Sound was crackly and cutting out. Also bad through the headphone port too. Decided to swap the speaker with a spare DS lite speaker to see if that was the cause.
At the same time I swapped out the shell for a new replacement shell.
Sadly didn't resolve my sound issue but the teardown guide was most useful as I have never opened up a GBA before. Now it looks all shiny and new with the new shell on it.
Don't forget to put the buttons back before you start screwing the motherboard into the case!