Surround speaker

I'm upgrading from 5.1 to 7.1.4. When it comes to surround music, does the music in the rear channels play in the surround speakers or the back speakers?
There is no one size fits all answer. It depends entirely on several things

- the multi-channel recording itself’s a bespoke creation that varies on how the recording was managed and crafted by the recording engineer

- a 5.1 channel recording on a 7.1 / 9.1 etc. system (and vice-versa) is a compromise playback; and thus dependent on how your multichannel AV processor actually decodes it, either by the default setting or with input by you.