Looking at GSMArena's audio quality review:
http://www.gsmarena.com/nokia_n900-review-421p7.php

Seems the frequency response is quite okay. Falls off slightly on high end but that's still way better overall than N95 8GB I have. (Which is frankly CR@P) At least it doesn't have a capped low frequency range which a lot of phones seem to possess.

I really would like on-par performance from the 3.5mm jack and according to gsmarena's review, it seems it might be decent. Just that the lack of equalizer will make it difficult for some earphones/headphones to provide sound as how it's meant to sound.

As long as it sounds as good as most 'okay' mp3 players with my IEM, I will be happy.