Distorted audio from iPhone through the bike's "radio" gear

I have a 2013 R1200RT and cannot find any threads with this problem.
My radio sounds great, both speaker and Bluetooth, as does the GPS through the Bluetooth headset.
When I hook up my iPhone 6 with the BMW cord to the bike and play music that is stored on the iPhone, the music is distorted, not horribly but definitely distorted. This does not happen with the same phone and music through a Bluetooth speaker nor, at least with my older phone, through Bluetooth connection directly to the helmet headset.
What would cause the distortion with the BMW cable? It is not an after market cable but is supposed to be an official, right at $100.00, BMW cable that takes the two wires into the one for the charge/connection port on the iPhone.
Wonder wheel only charges music but does not adjust volume. Only volume control is on the headset on the helmet.
Thanks.

The BMW cable for iPhones works or some, and is a real PITA for others.

When you toggle the SRC (source) button, you will see USB, which should change to IPOD once the radio has read the iPhone directory through the USB part of the BMW adapter. Depending on your library, it can take 10+ seconds to get ready to play. HOWEVER for many users, the radio will not recognise the iPhone and it will say USB and below it 'USB not found', or it might start to play music (clearly) and then abruptly stop with the 'USB not found' message. You will get distorted audio through the AUX source in this state.

For some reason, the BMW radio is pretty fussy about the tracks and playlists on the iPod or iPhone. I bought a BMW adapter when I first bought my 2011 RT in 2014. It would not work reliably with my iPhone 5. I gave up and returned the cable for a credit and reverted to a plain USB memory stick. Last week, I broke down and bought one again ( genuine BMW cable) for my newer iPhone SE after the bike got a software re-flash of all the controllers (including the radio I was told). I had the same trouble!!! Sometimes it would start to work, and then quit. Very frustrating.

Not to be outdone, I started to do some experimentation. I updated my old iPhone 5 with ONLY 4 playlists (BMW1, BMW2, BMW3, BMW4). I deleted the 'Purchased Music' playlist that Apple creates. So far it's working fine.

My guess is that there could be a track in my library with some screwball format, or non-ASCII character in the meta data that throws off the BMW radio. As time allows, I'm going to do some more experimentation with my SE and try to find what the problem is by process of elimination. If I find something definitive, I'll update the post.

Thank you for the reply

I look forward to hearing how successful you are with tracking this problem down.
I will give your suggestion a try and see if that works.

BMW cord

I was really excited when I bought my 2013 RT a couple weeks ago cuz it had a factory installed radio, not the ancient Sony Marine that I’d installed on my 2006 RT.
So imagine my shock & horror when I went to pair my iPhone 8 via Bluetooth, only to have it not connect. Reading the owner’s manual, I see that unlike EVERY OTHER VEHICLE made since 2010 or so, my RT is too stupid to just connect to my phone.
SMH.