Using Airfoil With Web-based Audio

Often, audio is playing directly in your web browser, and not in a separate audio player. Airfoil can transmit this audio to your AirPort Express just fine, provided you select the proper source. In these cases, you need to select your browser (likely Safari) as the source, as seen here:

Some examples of web-based audio for which you need to select your browser as the audio source include:
Pandora.com
Rhapsody
Last.fm
XM.com
Wolfgang's Vault.com
and many others.