I'm new to the site and to using MKVToolNix. Trying to use it to mux 2 discs, same format, into a single movie I came across the following issue:

Can anyone tell me why I would get this, I understand it might be to do with Audio Codecs, but can't figure out why.
Also is there a definitive guide anywhere to using MKVToolNix as I'm not sure how you use the merge facility etc.
Thanks