Acrobat Reader blocks my audio system, WTF?

I wanted to play a song (yes I have a legally bought copy from which I made the mp3) in mplayer and got the following result:

$ mplayer "01 - Adiemus - Karl Jenkins.mp3"



[...]

open /dev/dsp: Device or resource busy

After a few researches, I found:

# lsof /dev



[...]

acroread 32723 christophe 61r CHR 116,33 11606 /dev/snd/timer

acroread 32723 christophe 62u CHR 116,16 12023 /dev/snd/pcmC0D0p

An open document in Acrobat Reader was blocking my sound system. Why? No idea. I closed Acrobat Reader and opened it anew: no problem anymore.

For reference, it's a Ubuntu 8.04 on a PC, with a typical AC97 integrated chip. Package alsa-base is 1.0.16-0ubuntu4 and Acrobat Reader itself is 7.0.

EDIT1 30/04: I should say Adobe Reader, not Acrobat Reader, the former name.
EDIT2 30/04: The package acroread is version 7.0.9-0.0.ubuntu0.7.04+medibuntu2