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Hi and welcome to the community!
Sorry to hear that you have bluetooth issues after the update.
Have you also tried to update the firmware in your device in the car? I'm not sure if i found your product but Google led me to this page: http://www.diceelectronics.com/fw-update.php?manuf=Audiovox%20Corporation
As it works with our devices and your girlfriends car this sounds like it may be a compatibility issue between the device in your car and Android 5.0. An update of the device in your car may perhaps help.
Glad to hear that 
Yes, please let me know if the update helps or not.
So, I did update the Mediabridge to the newest firmware it didn't make a difference in sound quality....
A few days ago... I got a new update for lollipop build version 23.1.A.0.726 on my unlocked Z3 and I tested the bluetooth again with my car hoping maybe there was a change.... Still the Audio Quality is not what it was....
By the way I tested my girlfriends iPhone 6 and to my surpise the Audio Quality was not much better that my Z3 on bluetooth....
On another note.... My NWZ-A17 sounds the same way as my Z3 on bluetooth...
But I guess my point is that the way the bluetooth sounded PRE Lollipop was practically flawless....
I tested a few different apps to see if it was just the Walkman app but it's just the bluetooth in general....
Sorry to hear that the update(s) didn't help 
But you are sure that this is related to the update of the phone and that nothing else have happend with the unit in your car seeing as you also get poor audio with an iPhone?
hummmmm good point... well if i recall the audio quality with the unit pre update was better significantly better on the z3 than the iphone.... now they are roughly similar.... and like i said my high res walkman also didn't sound as good via bluetooth as my Z3 before the lollipop update... i wonder what was tweeked.... when i connect my walkman it will say that is connected via SBC codec... apparently there are different types/implemetations of the codec... some automatically default to a lower quality... maybe when the update on the z3 was done something in the default codecs were changed?
http://theheadphonelist.com/wireless-fidelity-making-sense-bluetooth-headphone-technology/
supposedly this article was done by with help from sony mobile actually
I'm afraid i can't see if there's been a change regarding this codec in Android 5.0 but i'll try to find more information about it.