I recently upgraded to sonar platinum from sonar 8.5 professional.
i REALLY miss these features (or... i can't find how to do them!)
 
1) play a single clip: in sonar 8.5 you could select a single clip in the arrangement window and, with shift+space bar you could play *just* that clip, in solo, automatically muting each other clip
 
2) quick mute/unmute a clip: in sonar 8.5, you could simply mute/unmute a clip by pressing "Q" (if i remember well). now you have to select the proper tool and then click on the clip.
 
3) where is the tap tempo function? i can see it only when i select the "insert tempo" feature. there is no "tap tempo" near the metronome icon in the main menu.
 
4) i cannot see any icon that indicates if the auto-crossfade between clips is on or off. very annoying!
 
5) I could select several tracks and automatically create temporary e.g. mute groups / solo groups / fader groups... now this is gone and it was very useful.
 
 

Not necessarily missing but transmuted by the alchemy that took us from 8.5 to Splat, via the Xs. For example, your 2 is achieved by "K" post-8.5. I found I had to try quite hard to forget everything I knew to use post-8.5 Sonar.
 

I have just checked all of these things.
 
1. Shift+spacebar is the default keybinding to "Audition current selection". So select a clip first then hit shift+spacebar. Should work as expected.
 
2.The keybinding to mute/unmute a clip is "K".
 
3. The tap tempo is right where you found it; "Project/insert tempo". You can assign a custom keybinding to it. I used "Alt+T".
 
4. Can't help much here. I don't believe there is an indicator but the function can be found in the options tab of the track view. This can also be keybound. (Keybinded?)
 
5. This is called "quick groups" select the tracks you wish to affect then hold down the cntrl key when clicking the desired function (solo-mute-fade) on one of the selected tracks. There is also a fairly new swipe function you might want to look into.
 
If you are not a fan of keybinding you can put up to nine of your most used functions into the custom module.
 
Hope this helps

thanks! it sound much better now! 
i am a fanatic of keybinding, i will try to find all what you have written on the italian version of sonar.
also, i will have definitely a look at "quick groups"...

Also check out the Smart Swipe feature.
 
I can't believe how often I used this now I have it

Don't forget Mix Recall!