Mmm only thing I can think of is trying the last version, as it looks you are still using 1.0 from the opening post. Maybe somehow it messed up.
The current version is 1.12 (21-Feb.-2010) and can be obtained from this link:
http://www.filedropper.com/maxpaynesoundpatchv112_1
You can just update the patchfiles and retry conversion(restore backup and convert again). If it does not work there is probably some files in the game directory that should not be there or the backup was damaged. In that case please uninstall the game, make sure the target directory is empty before reinstalling, and then convert using the latest version.
If you sure the conversion part did work and there's still no sound, maybe something else is going on...