I have a 2012 toyota Tacoma which I'm assuming has the same bluetooth setup as your lexus. I had my phone paired just as a phone first and it wouldn't let me pair as an audio device just like you. What I did was erase my phone from my trucks memory then set it up all over again. During the setup it asks if I would like to also use the device as an audio player and I just said yes. I think I must've missed that option the first time around. Now it works like a charm.
You can also take it to the dealer. They might be able to set it up for you.