Yes, SH5 sounds will work with SH4. The speech files are all .ogg and the sound files are all .wav.What you need to do is get a good free Sound Converter that lets you listen to .ogg files.
www.goldwave.com is a good one. You can use it to listen and compare sound files, then put in the one you want to create your own personalized sound set.
As far as I know neither OM nor OMEGU change any sound files. I could be wrong, though, and I have no way of checking at present.