Video files with 5.1 audio won't play in Premiere

Hello,

I've encountered a situation that doesn't seem to have come up elsewhere - I have a QT file with 5.1 audio on it, and when I try to play the video in Premiere, nothing happens. The space bar doesn't initiate the video playback. This is true on the timeline or in the source monitor. I am running 24.6.1 on a Mac Studio (Sonoma 14.6.1). My setup is stereo only, but I have set the 5.1 Audio Mixdown to Front+Rear+LFE; hoping that would fix the issue. It hasn't, nor does the video play with any other 5.1 Audio Mixdown config. The video plays fine in Quicktime Player and in Resolve. I've Googled this situation, but can't seem to find anyone else who's experienced the same issue. Any suggestions would be welcome! Thank you.