VLC

VLC is a free, open-source media player application, designed to play virtually all video and audio formats including DVD playback.

VLC is capable of playing DVDs using Apple TVs via Airplay. Apple's integrated DVD Player application does not work with Apple TV due to its copy-protection system.

Common Issues
No sound Subtitles
 * Check you have the correct sound output selected. To do this go to the menu bar and select Audio > Audio Device and select your preferred output. VLC occasionally needs restarting if audio stops responding after changing the output selection.
 * VLC supports subtitles. To enable/disable/choose subtitles go to the menu bar and select Video > Subtitles Track.

Specific Issues
Distorted / "tinny" sound
 * VLC has its own volume control in addition to the main system volume control. If you are experiencing distorted sound check VLC's volume level isn't set too high, setting the volume slider to around half way generally gives good results.