Many multi BLDers use letter-pair images for both edges and corners. The last cube can be treated as a single BLD solve - so you memorise it last and execute it first.
Although audio can be very nice for the final cube, it isn't that popular for memorising lots of pieces, since it can be difficult to remember a long string of complete gibberish.
I would suggest you try experimenting with routes/journey method. Think of place/s that you can visualise easily, then place letter-pairs images along the pre-determined route. Recalling is made easier if you're able to associate multiple images together. I only like to associate 2, while others might like 3, and some people like Mike can associate the images of an entire edge memo.
(Also, routes don't necessarily restrict you to using only images. Sentences can be incorporated into routes to replace images, Aron does this)
If you're really creative, you can make your memo flow like a
Rube Goldberg machine.
An example of this system is: CT + AH + JK + BT + TD = A CaT attack Adolf Hitler, in surprise he jumps back, knocking over the JoKer who is drinking from a BoTTle, he then falls on a ToaD.
Permuted but flipped/twisted pieces:
Depending on how many cubes you're doing, flipped/twisted pieces might be easy to memorise visually. Otherwise, you can memorise the letters of the flipped/twisted pieces. To avoid confusion between cycles and flipped/twisted pieces, you might like to place them at very end of the route, of maybe even in a different route altogether.
Reviewing:
Experiment with different memo review systems. Keep in mind, it's always reassuring if you finish off your memo with one massive review of all cubes (the last cube is an exception). Having a good review system is really important, as reviewing can be the longest phase during a multi attempt. (Phases are memorising, reviewing and solving)
It seems obvious, but too much review is better than too less. Not enough review results in forgotten memo and slower execution.
Sorry for the late response, hope that helped.