Finally, my no audio until call connects issue is resolved with 8.2
Yes! The issue with my Mazda3 was that calling with Siri would be done in Bluetooth Phone mode. But, while dialing, the Bluetooth Phone connection would drop and go into search mode. When it reconnects, it goes into Bluetooth Audio mode and sometimes picks up a few seconds after the call connects.
The latest update seems to correct this issue in that I can hear the ring tone and the call correctly reconnects in Bluetooth Phone mode.
Everything worked fine if I used the Mazda's built-in voice controls to handle the dialing. But, I would have to use the car's phone directory or recite the phone number.
The Bluetooth issue I had in iOS 7 was the turn-by-turn directions (and other non-streaming audio apps) getting cut off mid-sentence if I didn't have any other audio apps running at the same time. iOS 8 solved that issue, but had the new issue of not picking up the call audio quickly enough. Now I'll have to see if the turn-by-turn directions bug comes back.
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Aussie release 2014 Toyota Hilux
iPhone 5S running 8.0.2 (previous 8.0.1 only just upgraded and problems have been evident on both OS)
So for the first 6 months or so I have had no problems with the audio streaming or phone calls through the stereo. Then for no particular or obvious reason the audio stream has started doing some very annoying things. Apart from the radio having a return to set volume every time the car is turned on (Toyota design to apparently prevent me from blasting my eardrums) my streaming volume has now decided to have a mind of its own. Firstly, more often than not, I now have to pump the volume up to 45 or 50 (compared to 20 for radio) to even hear my music. Then when I get a notification it comes through at a stupid loud volume and the music stays at that higher volume and I have to manually reduce it. Secondly while I have the music at a reasonable level and get a notification the music (which has reduced for the notification) will stay at the reduced level and I have to pump the volume back up to hear it. These appear to be random events with no obvious precursors to the outcome. I can not duplicate either of the problems manually/on demand.
FWIW phone calls have not been affected and still function as designed.
I noticed this happening a couple of times. But, it's not a persistent issue for me.
Try disabling the Sound Check in the Music settings (or enabling the option if you currently have it switched off). This is supposed to work with iTunes's automatic level adjustments that try to keep the average levels for all songs roughly similar. Sound Check might be setting the reference level too low, and aggressively attenuating the Bluetooth output.
Another solution is to simply use an analog audio connection. Those are generally a lot louder than the Bluetooth audio.