I\'m trying to find a cross platform audio library that will have the following abilities (in order of importance):
Try out JUCE. It is like Qt in some respects, but much more audio-centric.I've been using it for some years now and it is well maintained and written.
You can take a look at GStreamer and FFmpeg.
EDIT:
Since you are willing to use Qt, you should definitely check Qt MultimediaKit, which is a part of the Qt Mobility project. Phonon is fading away because Qt is investing on MultimediaKit to replace it.
This example shows how to do simple audio playback. This example shows how to create a multimedia player. This example shows a more advanced music player, using Qt and QML.
I don't know if you are planning on using a framework like Qt. This has a library called "Phonon" bundled, which is also really nice. It is built on the corresponding native media framework, so QuickTime on Mac, Windows Media Player on Windows and GTK+ on Linux.
PulseAudio also looks promising without any framework.