Make audio recording work in mlv_rec/mlv_snd for 7D (back port some code from TL)

Created originally on Bitbucket by vyskocil (Vladimir Vyskocil)

Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.

A second try, this time I left out wave recording, keeping only needed changes to fix audio recording in mlv_snd.