Navman don't list the 8800 as being a compatible device (but that does not mean it will not work with the S80). Link:
Compatible Phones
However on the MP3 questions, yes and yes. (MP3 format is usable for both ringtones and message tones).
Sound quality through a quality set of headphones is quite acceptable. If you use the device as an stereo without headphones then I think you could call the sound quality adequate. (But this is subjective...

)
Hope this helps,
KR
Greg Myers