Hi!
  I'm new to this stuff and I've got no experience in DAW whatsoever, but I've been trying to transcript some musics from the sheet using the Cakewalk Sound Center synth. They sound amazing, thanks to the high quality (but limited) samples, BUT recently I've tried a new music in which included some repetitive notes. Some of the notes sounded kinda... off. Then I realized that if I delete de note before it, the note would start playing before the set time, which is the cause I've found for the problem, making some notes sound kinda dragged, because the sample starts playing on the right time but from the middle of it, not from the start, when the note before it have not been deleted.
  I've tried some solutions, like increasing the latency of the playback, or just import it in MP3 hoping that it's just a playback issue, but they were all for no avail. 
  I also couldn't find a post with a matter similar to mine, if anyone could help me with it, I appreciate it. Thanks for reading.
 

Welcome Bruno.
 
Midi is cool stuff and the Cakewalk Sound Center (CSC) is one of my favorite synths. The initial CSC comes with around 150 sounds but you are free to add more from the cakewalk store. The sound paks are $20 each and very in the number of instruments contained in each. I've added pianos and world instruments.
 
A few things about midi. As long as midi notes are NOT the same note in a pitch sense..... for example 3 middle C notes in a row, you shouldn't have issues playing the notes. AND.... remember that each midi note has an ON signal to start it and an OFF signal to end it. If you accidentally overlap the notes those ON/OFF signals get mixed up and the result is notes that don't play or start or stop when you expect them to do so.
 
Another issue it sounds like you may be having is what is called LATENCY. Where the notes don't play AT the proper time but are delayed to one degree or another. This is a result of the sound card and the driver. Factory cards exhibit this more than the after market USB musical interfaces.  It can be nearly impossible to work with midi on a factory card without pulling your hair out...... whereas the aftermarket cards make recording a breeze. My interface has no noticeable latency.
 
One workaround that may work is to FREEZE the synth track. Remember however,  that MC only allows 8 synths max in the project. So if you have more than that planned, it would be a better idea to bounce them and reuse that synth for the next track. The latency will still affect the synth track you are working on.
 
Some folks have had success with ASIO4ALL, and some haven't.
 
Anyway....welcome. Ask questions, and have fun.

Hey! Thanks for answering my questions.
Since I've read your answer, I've tried some stuff to see if I can better identify the problem. So I tried downloading the ASIO4ALL driver to see if could help me and freezing the track too, but it didn't. 
I'm also quite sure that its not quite related to accidental overlapping on the MIDI, because I'm working with an imported MIDI, and this issue is not present while using other synth like SI-Eletric Piano.
Also, I've noticed some really strange beahavior, that the problem disappers when I increase the tempo by 10 BPM, in this case, from 140 to 150 BPM. This made me think that this is actually a bug in the software.
Maybe I could just assume that this problem is just a issue on the software and try to ignore it just by increasing the tempo... although I'm quite unhappy to this... everysince I'm so annoyed by tiny details like this one. 

OK... so you'll want to be looking to get a dedicated interface. Those factory cards are not good to use with MC6 or anything from Cakewalk actually.
 
Focusrite, M-Audio, Roland(cakewalk) or Presonus are all good units to look at.
 
I use Focusrite. It rocks.

Hum... I'm still skeptical if this will really work, I  also intend to keep this just a hobbie, paying for a Focursrite seems too much of a expensive choice. But I'm thinking about buying a cheaper soundcard as it can provide a better sound performance at any kind of task. I hope this problem doesn't require an expensive sound interface to deal with...
 
Well thank you!

AHHH
I've just changed my preferences, the Driver mode was set on MME(32-bit) and I changed it to ASIO. And then the problem just vanished, and I kinda feel that the sound quality got better as well.
To think that just playing with the Prefs would save from such distress makes me kinda sad, but at least I won't have to buy a sound card anymore...
But I would also like to ask, why did it work? I'm still not completely aware of what kind of software change has avoided a hardware change...
 

Because you switched from the worst performing driver to a better one. Driver changes can have a huge impact on performance, as you have just demonstrated.

Yes.... if you have ASIO4ALL loaded, it needs ASIO as the driver.
 
I'm glad that worked for you.  I neglected to ask you to switch drivers to ASIO with A4A..... I thought that would be self evident..... I guess not.... (note to self in the future....) 
 
That should fix the issue for now. The interface would be the best option..... but if A4A works, use it until other issues make it necessary to get an interface.
 
Do not buy a new sound card that is touted as an upgrade..... gamer cards and such..... they all suffer the same debilitating issues as the factory cards and 98% of the time have  the same exact problems. Only a dedicated musical interface will solve sound card issues at the higher levels.
 
Glad you're up and running for now.