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If some application accesses the sound card though one of the lower audio layers (see Kernel layer), the sound card might become inaccessible and other applications might fail to open the sound card, or even hang.
OSS
You can see which applications are using OSS (or the ALSA OSS Emulation layer) with:
<pre> $ sudo fuser /dev/dsp* </pre>
ALSA
You can see which applications are using ALSA with:
<pre> $ sudo fuser /dev/snd/pcm* </pre>