How about this: What do you think about have a different color for the waveform graphic for WAV or AIF files? Keep the blue color for MP3 and other formats, but use a gold or other differentiating color for WAV and AIF files.
The idea would be to call these out as superior. It's a visual reminder that calls attention to the benefits of PLUS membership.
I always thought it would be good to annotate the waveform. It could be like soundcloud comments at a specific point in time, create a marker to say "this is verse 1", "guitar solo here", "chorus starts here", etc.
I really like this idea too... kind of a subconscious reminder. It would also give a cool visual cue to distinguish track types. The grey color of the wavform placeholder image would be perfect for that lower quality, just because it feels so generic. It's a little thing, but it would serve as a constant reminder, especially when working in mixed groups of "plus" and free members.