I have these tapes of my vacation in the summer of 2001, which I made on a microcassette recorder, and I have a couple questions.
1. What is the best way to make them stereo recordings? Are there microcassette recorders that output stereo? Maybe there's a plugin made for "Upmixing"?
2. What are the best plugins to clean up the sound (eliminate things like tape hiss, wow and flutter), upgrade the quality, and preserve the recordings as digital files?

I suggest to ask in specialized mailing lists and/or WhatsApp groups. iZotope Neutron and Ozone are accessible, I have no information bout RX7, which is the restoration package from the same company. But they have 10 days trial, so you can just test yourself.

There'a always the freebie, Audacity. May be worth  a try.

Wave Editors are the correct tools for ripping old tapes and LP's and removing noise. I have Sound Forge,  came with a turntable years ago,  and it does this almost automatically. Amazing results. I think this is now owned by Magix.
There's another thread here today with information about Wave editor's.
http://forum.cakewalk.com/Magix-Sound-Forge-in-Sonar-tool-menu-m3795086.aspx
 
 

I tried Audacity, but it made the quality worse. It made my voice recording sound like it was under water. I wonder, is there a wave editor plugin that would work with Sonar?

Annabelle
I tried Audacity, but it made the quality worse. It made my voice recording sound like it was under water. I wonder, is there a wave editor plugin that would work with Sonar?

 
This usually happens when noise reduction systems try to remove wide band noise that is quite loud in the mix, and the noise reduction algorithm removes too much.
 
The fault isn't audacity, it's a combination of your source material and audacity's noise reduction algorithm.
 
If it's just your voice on its own (i.e. a noisy mic / mic pre), you could try noise-gating the silent parts, then use either a graphic EQ or a combination of parametric EQ's to try to remove the worse offenders.
 

msmcleod

Annabelle
I tried Audacity, but it made the quality worse. It made my voice recording sound like it was under water. I wonder, is there a wave editor plugin that would work with Sonar?

 
This usually happens when noise reduction systems try to remove wide band noise that is quite loud in the mix, and the noise reduction algorithm removes too much.
 
The fault isn't audacity, it's a combination of your source material and audacity's noise reduction algorithm.
 
If it's just your voice on its own (i.e. a noisy mic / mic pre), you could try noise-gating the silent parts, then use either a graphic EQ or a combination of parametric EQ's to try to remove the worse offenders.
 

It's actually a combination of voices, airplane and car background noise, and airport background noise. However, the offender is the hiss and hum of the tape recorder, as well as "wow and flutter". Since I'm blind, spectral editors would be hard for me to use, as they're graphical, instead of buttons, dials, and sliders with accessible text.

Annabelle

msmcleod

Annabelle
I tried Audacity, but it made the quality worse. It made my voice recording sound like it was under water. I wonder, is there a wave editor plugin that would work with Sonar?

 
This usually happens when noise reduction systems try to remove wide band noise that is quite loud in the mix, and the noise reduction algorithm removes too much.
 
The fault isn't audacity, it's a combination of your source material and audacity's noise reduction algorithm.
 
If it's just your voice on its own (i.e. a noisy mic / mic pre), you could try noise-gating the silent parts, then use either a graphic EQ or a combination of parametric EQ's to try to remove the worse offenders.
 

It's actually a combination of voices, airplane and car background noise, and airport background noise. However, the offender is the hiss and hum of the tape recorder, as well as "wow and flutter". Since I'm blind, spectral editors would be hard for me to use, as they're graphical, instead of buttons, dials, and sliders with accessible text.

I think cleaning up this scenario would be a challenge for any program.
 
What you could try though, is first using an EQ with a narrow band to remove the hum. It'll probably be 50Hz or 60Hz depending on where it was recorded.
 
Once you've done that, a low pass filter might help to remove the hiss. You could try using an enhancer to try to add back in some "fake" high end.
 
I'm unaware of anything that would remove the wow and flutter though, unless you could somehow automate an EQ to try to counteract the effect. If you've got a hardware slider controller, this may help since graphical interfaces are no good for you. You could then practice moving the slider in time with the track before recording the automation. I doubt if it would remove the wow and flutter completely though.
 

Audacity would have been my last choice,, as said try Sound Forge. 
 
The plug in that does this best is isotope RX but your talking $$$$ for the pro versions. RX7 elements is affordable @ $129 and there are probably sales.  
https://www.izotope.com/en/products/repair-and-edit/rx.html
 
I tried the demo once and it was super complicated to figure out. 
 
I have most of the popular Wave editors on hand including Audacity.  The only good quick and dirty thing I've had success with was Sound Forge. Hiss and pops and crackles are much easier to fix as there's been lots of work put into removing those.  But removing traffic/ background noise is a job that a very highly trained engineer would be required to tackle.  
 
 

Cactus Music
Audacity would have been my last choice,, as said try Sound Forge. 
 
The plug in that does this best is isotope RX but your talking $$$$ for the pro versions. RX7 elements is affordable @ $129 and there are probably sales.  
https://www.izotope.com/en/products/repair-and-edit/rx.html
 
I tried the demo once and it was super complicated to figure out. 
 
I have most of the popular Wave editors on hand including Audacity.  The only good quick and dirty thing I've had success with was Sound Forge. Hiss and pops and crackles are much easier to fix as there's been lots of work put into removing those.  But removing traffic/ background noise is a job that a very highly trained engineer would be required to tackle.  
 
 

I don't mind so much the traffic background noise. It's the tape hiss and motor hum of the recorder that I'm trying to get rid of. And I'm trying to turn the recording from mono "dictation-quality" (the quality of most microcassette recorders I've worked with) to stereo studio-quality. Is there a way to achieve that goal?