I frequently play long playlists on my Nexus 9 tablet, while it sits unused in any other way. Since updating to Marshmallow, the audio suddenly slows down to (at a guess) 50% normal speed after 90 to 120 minutes of playback. The audio file needs to be stopped and restarted to restore normal speed.
What I tried:
My first thought was that Marshmallow's new energy conservation system, Doze, was likely to blame. Disabling it for the media player* made no difference, nor did
playing files stored locally or on the LAN or
using a wide variety of other media players.
As this behaviour only started after updating from Lollipop to Marshmallow, I still suspect Doze is somewhere at the root of this. That said, Google produced no obvious mention of anyone else running into this issue and it seems unlikely no one else would have bumped into this.
The tablet is not rooted and both the original Lollipop and the current Marshmallow were auto-updated from Google.
*(Options\Battery\...\Battery optimisation\All apps)