Bluetooth quality does seem to suck pretty bad. I end up streaming which I'm sure is part of their goal but hate that it's not Spotify. Like gfb107 said, you can load a USB stick with music, or some MP3 players (my iPhone does not load through the USB, doesn't mean there's not a way I haven't seen to do it though)
Once you plug a USB stick in with songs on it, it will show up as an option in the music screen. I just don't understand why no one else loves slacker as much as I do... Just click the "listen" button on the right side of the steering wheel and say "Play Eye of the Tiger by Survivor" and it starts playing the song. It also has several stations that are great - 80's, 90's and today is my favorite. I use a USB stick in all of my other non Tesla vehicles.
Slacker doesn't work anywhere near as well as it used to. I often get "pauses" for no reason and the tell it to "Play Eye of the Tiger" barely works anymore. Songs that I've been able to tell it to play numerous times it can no longer find. Yet I will hear the song playing randomly, so it's obviously still within the Slacker system.
Also, because of the low bitrate of Slacker (Tesla won't allow the high quality transmission) the audio quality really isn't that great. And finally, I spend A LOT of time in the car, and although Slacker isn't anywhere near as bad as Sirius/XM, I'm getting tired of hearing the same songs over and over.
I love the audio quality of Slacker - have you played with the equalizer settings? I turned the bass all the way up and the mid range almost all of the way up. I also turned down the treble. I guess I am just different that way - I spent two years of my life in my twenties eating peanut butter and jam sandwiches every day for lunch and dinner. I could play Freeze Frame by the J Geils Band over and over and over again...