I label this one "KDE" because the issue I've had tends to be KDE's fault. However, I've seen other references to Ubuntu and Gnome having similar problems. In those cases, the blame was levied against something called Pulse. I don't use it, I don't know what it is, but it has seemingly caused a handful of media problems.
Now, if you sound stops working on your computer, check the following things first. Are you absolutely sure that no portion of the sound is muted? If you are playing most files, you have to check master channel, sometimes separate output channels, PCM channels, and maybe others. If any of them were muted, and some programs will do it almost unexpectedly, then the sound won't work. It doesn't take long to check, and is surprisingly common.
Second, do a quick check to make sure something like Firefox isn't taking over the sound. Another possible culprit I have found include Frotz/Gargoyle (for IF gaming). Sometimes, these programs will take up the sound card without actually playing any sound, and so will confuse you.
Let's assume you have done those two quick checks. Try running something like Mplayer (or any sound generating device in which you get output). When my sound stops, or when a program is taking up my sound playing abilities, I will get a message like "Can't open audio device /dev/dsp: device or resource busy". I've heard that /dev/dsp is being deprecated about now, so this might not be true for long, but still, this is the error message I get. 9/10, the answer is obvious. Find the obvious program (music player, whatever) that is taking up the sound. Then, find the not so obvious app taking up sound, say the Firefox example above. A couple of times, though, I was stumped because nothing that should have sound output was playing.
I played around and found this solution. First, type in one (or all) of these three commands:
Chances are, you will get nothing back, but you will usually get whatever program is conflicting. The weird thing is, I have found, several times, that KDE will have given one of its native apps (Ktorrent, KGet, etc) control over the /dev/dsp and that is hosing the works. I don't know why it does this, or how to stop it, but it does. You simply kill that program, and you should be good to go.
Written by W Doug Bolden
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