Same regression here on macOS 27.0.1 (26A434), with two DisplayPort displays, so it isn't specific to HDMI + DP: MacBook Pro 16" 2021 (M1 Pro) 2 × Dell S2725QC, each on its own USB-C port (DP Alt Mode, no dock) Each display's audio works alone, never both at once. The display whose link was negotiated last (boot, replug, refresh-rate change) gets audio; the other fails with the same 'stop' (1937010544) / "could not establish a timeline" error, so a Multi-Output Device with both never starts. It happens below Core Audio: when one display's DP audio DMA starts, the other's stops (its DCPAVAudioDMADelegate TransferInformation counters in the IORegistry freeze, and the kernel logs DCPAVAudioDriver … stopIOEngineGated: _hostTime=0). Different sample rates per display, renegotiating both links at once, restarting coreaudiod and rebooting don't help. Partial workaround: changing a display's refresh rate (e.g. 120 → 60 → 120 Hz) gives that display the audio back, at the expense of the other. Filed as FB25041393.