I've played them in MPlayer without much trouble. Quicktime streaming can be a bit of a pain though as well as other streams. I found an excellent extension for Firefox called MediaPlayerConnectivity that solves a lot of headaches. It lets you open a video stream into an external player...MPlayer in this case.

The MPlayer website is at www.mplayerhq.hu and there are several sites out there that help you install it. I run Fedora and I found an excellent walkthrough here:
http://www.mjmwired.net/resources/mplayer-fedora.shtml

It might not be the same for your distro, but it should point you in the right direction.

vgui
mov (quicktime files) are no problem. got a few of them and they work fine. might have to get a codec for mov files - google for mov codect linux or something. i use xine and i don't think i've had any problems playing anything.

m4a - not sure what these are. - might just be being stupid tho. again just google for it and see what you find.