Help! NO VIDEO PLAYBACK unless connected to external display. (2018 MBA on 10.15.4)

2018 MacBook Air on Catalina 10.15.4

Videos only play when connected to an external display via HDMI or AirPlay. No streaming service (Netflix, YouTube etc.) work in Safari or Chrome and even local video files won't play. I can scrub through them and see the frames, and videos even play correctly on the built-in display, as long as also I'm connected to an external directly or via AirPlay. I first noticed the problem after disconnecting from my TV, which had be connected directly via HDMI.

What I've tried: 

• Multiple restarts
• Booting into Safe Mode
• Testing in alternate system user account
• System Update (now on 10.15.4, problem persists)
• Resetting SMC and NVRAM
• Killing the "coreaudiod" process (Read that solved some playback problems)
• Reinstalling the system software 
• Run disk first aid

Not sure what else to try at this point, and I'm concerned it's hardware-related. Any ideas would be greatly appreciated! Thanks.