tgeo:

For step 1, look at line 6 to see how to get a list of filenames based on a pattern, and look at the code for step 4 (ca line 37) to see how to delete the files listed.

For step 2, read up on sox to see how to do the mixing (I've never used it), and look at lines 14 and 20 to see how to call it.

...roboticus

When your only tool is a hammer, all problems look like your thumb.