Just came across software called MSX Synth – it’s not specifically for the CX5M but you can use it to play around with the internal Yamaha sound chip: http://paxangasoft.retroinvaders.com/msxsynth.htm

It’s very nice to see how Paxanga has used the graphic possibilities of the MSX1 to make an advanced interface for the PSG sound chip. You can see a video here how it works:

Paxanga soft claims it’s still in development but I think it’s worth a try already.