Same situation here, years ago, so I may have forgotten some of the moment.
You have both discs in, but #2 is digital, which means that music won't work if it loads from #2, because for some reason (explained somewhere else) music doesn't work with the digital iso's. The menu, though, can load from #1, and so that's what happened. It loaded the menu and its music from the actual disc #1, but the real game part requires #2, and therefore can't have music unless you find a physical disc #2.
