>I'm working on a C# Skype application. Skype won't dial if the local audio is in use.

I don't see what you can do in that situation except trap and report the problem from the C# app that's using Skype?
Even if you could, It's not going to be practical to go round knocking any app that's using the PC speaker on the head.

But does the application actually require incoming sound?
If not you could maybe set Skype to use a different non-existent setting (e.g USB) for speaker audio.
Or, in the Skype configuration settings, I see that you can list multiple Speaker devices and Skype should use the first available....

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>>>Is there a way in C# to determine if the PC's speaker is in use?
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>>It doesn't answer your question, but in my experience PC audio multi-tasks i.e. can play more than one sound simultaneously. For example, if I'm playing a video with sound, I still get the e-mail notification sound when a new e-mail arrives. They play together, one doesn't interrupt the other.
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>>Maybe you have a requirement that your sound plays only when no other sounds are playing. What you really need is an exclusive lock on the audio subsystem; simply knowing that no other sound is currently playing isn't good enough, because that's no guarantee that some other sound won't start playing during the playback of yours.
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>>I don't know if it's possible to get an exclusive lock. I didn't see anything relevant with a Google search of ["pc audio" "exclusive lock"].