Well... I've spent some time on this and it's far from straightforward, to say the least. I installed SoX and converted a .WAV file to 8SVX and it didn't work, however it was an 8bit WAV file, I might try 16bit as apparently that works. The SoX instructions are, convoluted to say the least - I can find the command for compression factor, but I can't find the entry telling me how to use it (it states 'See the description of the file formats that use this option in soxformat(7) for more information' but I can't find the soxformat option. Still working on it. I'm wondering if the fact I'm using virtual SID's using address sniffing might have something to do with the noise during playback. I think I'll make a lead to use the real SID.