My problem turned out to be mechanical in nature. One of the mounting tabs holding the top cover in place had broken off and fallen underneath the left IO board (I thought the part had fallen out of the computer). Removing the tab fixed the boot issue, but the speakers still don't work; external audio, however, does work. I think the tab may have short-circuited something and fried the speaker circuit. I'm just fortunate that nothing else appears to be damaged.