Sorry I`m not at home at the moment, and here I got no Monitor Headphones and Speaker for checking, I let you know in 2 Days if the quality fits. But the others are right, when you only work on digital Level for Recording and encoding to mp3, the soundquality cannot be better as the source is, so VBR encoding is the best way if you want something between small filesize and better quality on heavy dynamical parts. So it makes sense when you got gamemusic with more silent parts and loud parts, so VBR decreases the quality everytime where the full bandwith is not needed for a good result. On a Chiptune like those from Battle Squadron, VBR might be not so good, course chiptunes are flat sounding and allways push the dynamic to the maximum, there CBR encoding is better. Normaly I would say something between 128kbit and 192kbit mp3 quality is ok. Course The Paulachip can only play 28khz resolution maximum depending on the sample playback rate, this is far under what 128kbit can handle. So better trust your own ears when hearing.
Some gametune sounds are very Lofi, no matter what you try to tweak. There It makes no sense to use a higher bitrate in my opinion. So use 128kbit minimum for all, and higher when needed, my opinion. Better spare some webspace instead whenever you can for more sounds :O)