As always, make sure you have completely rebooted the phone by selecting restart. Just in case the Bluetooth service went haywire or something.
Does the Subaru have an A2DP (Bluetooth audio) capable radio? Its possible the radio supports voice only. Check the radio manual. Bluetooth audio will be an explicitly mentioned feature and often is its own mode.
If so, go to the Bluetooth settings on the M8, press the gear icon next to the name of your Subaru radio profile, and make sure both Phone audio and Media audio are selected. If Media audio isn't there at all, the radio isn't reporting that it supports A2DP to the M8. If you know for a fact that the radio supports A2DP, check for a firmware update for the radio from Subaru or the radio manufacturer if its aftermarket.
If its there, but there is no sound, make sure your radio is on the Bluetooth audio setting (check your car manual). And turn the Bluetooth Media volume up on the phone.