I have a circuit that I am currently designing using a 4 layer PCB (signal, ground, power, signal). The PCB has rather a lot of vias to route the signals around the board (there are size constraints, meaning I can't space the ICs as much as I would like which would make it easier to route the signals without vias). The circuit is primarily an audio intercom system, with the vast majority of signals being of audio frequency. There are only a couple of ICs that have high (10MHz) signals and these I have been able to place close together, i.e. reduce signal trace length.
My question is when should you consider adding an additional signal plane? Is there a point at which it becomes better to add an additional signal plane (e.g. going to 6 layer board instead of 4) rather than use a lot of vias? Is there a frequency of operation above which it is better to do this?
Edited to add PCB images: