Could you add a Volume slider to the App Interface, not on each music clip, but one that controls all audio previewed when hovering over a clip.
I will preview a track by playing my sequence, then hovering the mouse over the track I am considering. The Pond5 audio plays simultaneously with the sequence.
It is a super fast way to test multiple tracks without even downloading the preview. The only problem with this technique is that web audio is much louder than my sequences so its tough to hear the mix. A volume slider would make this easy to customize.