This is not something you can control in the Movie Menu workspace. You'll need to adjust the audio on the original music clip.
There are a number of programs that can do this. The free download Audacity may be your best choice. Just lower the audio level based on the audio level meters in the program and then save the file as a new WAV file.
Thank You very much for help!
What is the file extension of the audio that you want to use for the background music for your Premiere Elements 12 disc menu? If your issue is only with volume and not format, then......
Although Audacity is a fine audio editor, you can also achieve your volume adjustments goal in Premiere Elements 12.
Import your audio and adjust the volume via Adjust Tab/Adjustments Palette/Volume Panel expanded. (Premiere Elements has audio meter also - See Window Menu/Audio Meters).
Then export the file Publish+Share/Computer/Audio with Presets = Windows Waveform.
(Just in case note, when you get to Publish+Share/Computer, you need to scroll down to Audio which is at the bottom of the list of choices there - See thin scroll bar to the right of the choices that start with Adobe Flash Video, MPEG, and AVCHD.)
Thank You A.T.!
My annoying problem was with the volume level only.
I managed to get it right using your instructions.
Great, great news. Glad the suggestion worked for you.
Your follow up is important for those who find themselves in a similar situation. Consequently, many thanks for letting
us know the outcome.