Have you tried the plugin from rarewares?
If that plugin doesn't play back the lossless m4a files, please deinstall the rarewares plugin and install my plugin from this thread:
My plugin needs Quicktime 6.5.1. It should be able to play lossless m4a files, but I haven't had a chance to test that yet. Please write back if it works - if it does, I will update the plugin (it probably still contains the memory leak I've fixed in my m4p plugin yesterday). It would be cool if you could also test if visualisations work correctly. Thanks!
: I just saw Gonzotek's post. If the codec is a proprietary Apple codec, the rarewares plugin won't work. My plugin should work, however, and it should support visualizations, DSP plugins, burning, etc. because the lossless files are not FairPlay-protected (unlike the m4p files from the iTunes Music Store)