Hmmm... an interesting range of expeirences regarding the sound quality of the MP3 player.
I use mine with a pair of in-ear Sony headphones. Can't remember the model number but they were about �35. I've never picked up any GSM interference noises or similar. I just hear the music.... apart from in the over-long gaps between songs which are indeed rather noisy.
Although the maximum volume is unlikely to burst my eardrum, I find it to be plenty loud enough, and any louder would probably have a seriously negative effect on my long-term hearing... and annoy my co-workers.
Most of my MP3s are ripped at 160kbps (and some higher and some lower) using iTunes. Depending on your ripping software, you may be able to rip your MP3s at a louder volume.
The sound quality is hardly audiophile-standard, but it's good enough. It lacks definition (and volume) compared to my iPod nano, but I don't have a particular problem with lack of bass. (Interestingly, my iPod nano is also much louder and clearer than my old 4G full-size iPod).
And of course, personal opinions on the sound quality of any audio system will always be very very subjective. It depends on the style of music you're into (I onced owned a very expensive HiFi amp that was great for classical but not much use for anything else), what you're used to, and the quality of the other equipment in your sound chain... and that includes your environment and your ears!
If the N80 was just an MP3 player, it would be sadly average. But for a device that's also a voice/video phone, still/movie camera, synchronisable PDA, FM Radio, Web/WAP access device, eMail client, pocket gaming device, portable memory vault, Remote Control AND GPS navigator (once TomTom sort it out) with more forms of connectivity than any device in its class so far, the N80 is a bl**dy good MP3 player.
And if you don't like it, I'm sure there'll be something better along in two months. Then something even more better two months after that. And then... you get the idea.
SL