[nevcairiel]

The most likely case is that your AAC audio file decodes to a different channel layout then you might expect, since LAV actually tries to obey the layout information in the file, while older decoders like ffdshow just ignored such info and put it in a generic 5.1. That may be a case of a badly encoded audio stream, or just needing some tweaking in the audio renderer configuration to map it to the speakers you actually have.

Without a file at hand or some info I can actually go on (like maybe the output pin info of both LAV Audio and another decoder), its impossible to say anything else however.