How far away from the router are you? Are there many walls?
I had trouble streaming MP3s on my network due to poor wireless signal until I got myself a homeplug wireless extender:
http://www.cableforum.co.uk/board/87...-extender.html
Not cheap (£100 5 months ago), but it works a treat (and was a doddle to set up). My router is in the study, and I basically have a wireless access point in the lounge that blasts out any of my neighbours networks. I've not had any problems with the wireless signal since buying this kit. It's like you're sitting right next to the router. It's the bee's knees really.