There are several possible reasons why this can occur, the most common being:

1. The audio hardware (sound card) is not setup correctly for CasaTunes
2. The location where the music resides is not accessible
3. You are trying to play songs that are not compatible with CasaTunes (most often iTunes protected music)

To determine whether these are the cause of your problem, please check the following:

1. The audio hardware (sound card) is not setup correctly for CasaTunes

a. Check whether your CasaTunes Player sources are correctly setup

Start the CasaTunes Setup application
Click on the Sources tab
Verify the type of source is setup as a CasaTunes Player
Verify the audio device selected is correct.  For the XLi/e hardware it should be “Speakers (CasaTunes Mutli…”.  For other sound cards on Vista, select the audio device that starts with “Speakers ...”
Verify the speakers you are using for this source are correct.  It should be 1&2 for the first source, 3&4 for the second, etc.
Verify the cables from your sound card to your audio matrix switch match the CasaTunes setup.

b. If you are not using the CasaTunes XLi/e hardware, you must verify that your audio card is configured as a multi-channel card.

In the Control Panel select Sound
Select Configure to configure the speakers for the device you are using with CasaTunes
Select 5.1 or 7.1 speaker surround configuration depending on the type of audio card you are using

c. If you are using Windows Home, 2003, or 2008 Server

You will need to set an application setting in the CasaTunes configuration file
Stop the CasaTunes Windows Service using the Services manager
Start Notepad
Open the file C:\Program Files\CasaTools\CasaTunes2\CasaTunesSvc.exe.config
Find the application setting OverrideSpeakerCount
Change the value from false to true
Restart the CasaTunes Windows Service using the Services manager

If you changed the audio setup for your computer you should restart the CasaTunes Windows service using the Services manager.

2/3. The location where the music resides is not accessible or the song is not compatible with CasaTunes

To find the location of your music within Windows Media Player:

Locate the song and click on properties - the full path of the song will be displayed.
If the song is located on a different machine, make sure the UNC path is accessible (i.e. that it exists and that you have the correct security access)

To find the location of your music within iTunes:

Locate the song, right-click on the song and select Get Info.

The path to the song is displayed in the Summary tab.
If the song is located on a different machine, make sure the UNC path is accessible (i.e. that it exists and that you have the correct security access)

The Kind of song is also displayed in the Summary tab.  Verify you are not trying to play an iTunes protected file.  If this song is protected you can upgrade the song to iTunes Plus (which CasaTunes can play) via the iTunes store.