When FreeAmp is installed, it detects and then displays the names of the .pmo files - the Physical Media Output (sound card drivers) that are installed on your computer. From the Preferences Plugins window, you can choose which sound card driver you want to control the audio output for FreeAmp.
Soundcard.pmo is the default soundcard driver.
Esound.pmo supports the Enlighted Sound Daemon.
Default ALSA Output Device supports the Advanced Linux Sound Architecture.
Note: Depending on your system's configuration, other options may appear in the Preferences Plugins window.