I had a very similar problem to what you describe here and I can confirm that in my case it WAS Skype.
What was happening to me was that the bottom speaker was not working and all of the sound, including iPod music was coming out the ear speaker. After reading your post I looked at what was running and Skype was active. I shut it down and all my issues were solved.
I hope this helps.
After running Skype 2.1.0 and the person puts the call down, all sounds only come out of the ear speaker. Using the multi-tasking shortcut and closing Skype results in the normal functioning of all the speakers.
Forum post on the Skype forums can be seen here: http://forum.skype.com/index.php?showtopic=659433
Thankfully they are aware of the issue:
Tero Hurskainen wrote:
It seems that the new backgrounding behaviors still need some tweaking by both Apple and 3rd party developers. The interactions have suddenly gotten quite complicated.
We suspect that this is the work of a volume control flag which is handled in different ways in different apps. Some scenaria produce this effect and we're looking for a proper way to make it behave. It may be something that Apple needs to set the rules for in a different way.
We're on it and will change it for the better in the next release.
Message was edited by: A2D
I got new Iphone 4 - no skype and sound dissapeared just in the shop on testing, they changed to another one and when tested again - it make one melody then got silent again. but luckily after restart it worked... when I come home it got silent again - I just restarted - doesnt help - I will try more.... but I think this is not normal