It could be a 'difficult' form of Matroska file. FYI Matroska is just a container like MOV or AVI, and some mkv have the video encoded with Real Video Codec (RV9 RV10). Perian+QT can't decode this, but VLC will play .mkv files just fine on OS X, as long as they don't use H.264!
It's a bit of a lottery until Matroska decide to support the Apple platform. I have found this:
which may help.
But there may be another workaround: 1. Get Info the mkv file
2. Change to extension to wmv. Save as wmv and that could work better.
Or this link may be helpful:
Recently, a poster (ZachP14) suggested this:
"There's a program called Plex that runs .mkv files awesome on my 2.0 ghz macbook."
Maybe worth a try?
More information here:
Another solution is to download the free transcoding program Handbrake for Mac (you will have to download an earlier version that will run on PowerPC and Leopard) and use their AppleTV default.
Transcoding can be horribly slow on a G4 PowerMac, but the result will play flawlessly on QuickTime.