Hi Julian!
If you are going to add MP3 support to JUCE, please consider the following library:
here: http://mad.sourceforge.net
or here: http://www.underbit.com/products/mad/
It produces the excellent sound quality (if compiled with quality flag) and it’s easy to integrate it into the JUCE framework.
But of course you’re not allowed to actually use it without paying royalties to the Fraunhofer institute! Thanks for the link!
between ogg and flac, I can’t imagine what else people need - besides interoperability with pretty much every crappy media device out there. :roll:
Still, thank ramen for SqueezeBox. 
yeah, but it’s not a portable if that’s what you are thinking:
The server side is open source, and can be used without a hardware squeeze device. I use it to listen to my CD collection (all FLACed on a large server box) when I’m at work, in addition to powering my squeezeboxes at home.
There’s support for almost every format except closed DRM proprietary ones like iTunes.