First off, thanks for your constant updates zx-81 :)
In the PC STELLA builds, there's a command line parameter of '-clipvol [0/1]' that supposedly gets rid of the sound clipping.
Whenever you're bored, could you take a look at how they handle that? If it's easy to implement and use on the PSP version, it would probably also fix the major sound clipping/crackling problems that show up in PSP7800 and PSPATARI.