Sometimes Rex missed ROX-N over at Star Tours. Sometimes he didn't. This was one of those times.
In Oga's Cantina, he was DJ R-3X and free to pursue his passion - playing dance music to hoards of inebriated life forms who didn't seem to care very much about it.
It was a shady joint frequented by smugglers and thieves and he often overheard stories. Occasionally, he'd see violence.
Which is why he had stopped his music and was looking on, practically in glee. Although he was supposed to be the entertainment, he knew a drunken bar fight was the real entertainment.
R-3X used to be RX-24 from Star Tours, the terrible captain who crashed a lot. On that ride, he was known as Captain Rex. Then, according to canon, he crash landed on Batuu, was fixed up and sold to Oga, proprietor of Oga's Cantina, and became DJ R-3X. He also has an album coming out - in the real world that is.