You have the perfect memo system, as audio edges and image corners are considered to be the best for 3BLD
If you have trouble in recalling your edge memo, then you should probably give more time to memorize your first letter pair, before moving on to the next.
The main problem you are having is because ,what I think is that you are trying to memo faster than your current capability.
I remember I had the same problem, about 5 months ago when I had just started out the audio memo, I would think that my audio memo was strong , but as soon as I started to execute my edges, I almost forgot my entire edge memo. Then I started to give to more time to memorize my audio pairs and eventually with practice I could do it in one loop without any recalling issues.
Audio memory is extremely short term, so getting used to it requires a lot of practice.
Currently, I can memo my edges with great fluidity, but one more problem I have discovered is mixing up the audio string.
By mixing up, I mean that the audio pairs sometimes sound alike, and I couldn't differentiate between them. For instance, audio pairs starting with X or J or pairs ending with E or I had the same sound.
So I am currently working on these issues which you might also have.
So,my final advice is just to slow down your memo time and memorize more concretely and identify your mistakes.
If you have anything else, please let me know.
Hope this helped .