RELENG_7 problem with playing DVDs

[[LoN]Kamikaze]

Using mplayer to try play a DVD my errorconsole is spammed with:

ata1: FAILURE - non aligned DMA transfer attempted
acd0: setting up DMA failed

The mplayer just hangs around in physrd status and cannot be killed (even by
kill -9). The error message gets posted until the system is shutdown. The
mplayer process also stays alive for the whole time. The only way to open the
DVD-drive after attempting to play the DVD is to reboot the system.