Could be any of a number of reasons with Pandora. If you can actually get into Pandora, choose and play a song but there is no audio as you describe then it's probably Pandora being ornery. Especially if you can exit and re-enter and the audio kicks in. Sometimes, the HDUI can be a bit sluggish so doing an HDUI reset might help. With your remote try a thumbs down, thumbs up, play, play. After the "ding", the HDUI will reload and repopulate the icons. Expect Pandora to do things like this, especially if you play it for long periods. Every now and then a pop up will ask if you are still listening or not. Unless you happen to be sitting and staring at the album art, you'll probably miss it. Pandora will assume you aren't interested anymore and just stop playing. Usually takes exiting and re-entering again. I find that I can (usually) avoid this by zooming in to a song every now and then and thumbs up or down a currently playing song.
Then again, I've also had to kick myself because I either turned down the volume or hit mute for a phone call or something and then wondered where the audio went.