For anyone that listens to MP3's on their K750i, I highly recommend the K750i to 3.5mm stereo adapter. Link here:
http://www.mobilefun.co.uk/product/7265.htm
I got mine delivered with 48 hours and price is better than the price you would pay on E-Bay the last time I checked.
These sell out very quickly as they're currently showing a lead time of 1-2 weeks. Supply a decent set of headphones and the sound is pretty awesome.
I ripped one of my CD's using Exact Audio Copy using default LAME MP3 settings and listened to a few tracks on my Sennheiser PX100 headphones. The sound is very, very good for a mobile phone! Whilst it cannot compete with my iRiver H140, the K750i delivers a warm, rich sound with a soft edge. It's very relaxing and the music can be enjoyed for extended periods without causing fatigue.
The supplied K750i earbuds are just so poor in comparison and do not do the audio capabilities of this phone justice.