When I try to play a sound I get an error message that says another application is playing.

In Mp3 Audio Mixer, click on the File menu and select Preferences. On the Playback tab, click on the Playback Device setting and choose the driver for your sound card. Do not choose "Default Sound Device" or any other option aside from your specific sound card"s driver.

If that doesn't fix the problem, check and see if there are updated drivers for your sound card on your computer or sound card manufacturer's web page.