This is a trick I heard from a friend at one point in time. This kind of shelving isn't meant to support a whole lot of weight, but seems to be working well enough for me. I have the speaker sitting directly over the wall support.

Take an inexpensive clipboard and wall mounting bracket (I got these for about a total of $3 at the local surplus store).

Hot glue the bracket to the bottom of the clipboard as in the photo:

Then mount the shelf to the wall