I got these new speakers for my computer - Bose. When I play MP3's or CD's at high volume the volume fluctuates....
I have the "mix" control on the speaker set in the middle - as well as the other settings....
Any idea why that happens?

got the actual volume control for the speakers themselves at about middle?

set your computer sound to about low midrange and control the volume with the speakers.

What sound card are you using in the pc?