By default, ProSelect uses your topmost music group (in the Setup Music window) at 25% of maximum volume.

If you have setup you own music in ProSelect (under Slideshow menu --> Setup Music), then drag your required ambient music group to the top of the list to set it as the default ambient music for each album file (.psa).

You can set a different ambient music and volume for an album file (.psa) by opening "Setup Ambient --> Setup... "  (under Album menu)  and make any changes (including changing the music group) and click Save. In doing so, these settings are then saved with each album file (psa), so you can pre-setup different ambient music for each client and used whenever that album file is opened.

You will find this and other helpful information under the sections "Using Ambient Music" & "Setting up your Music" in the ProSelect Reference Manual (quick access under the Help menu in ProSelect).