MP4 export strips audio into a separate/unplayable AAC file in Media Encoder

Hi,

I am encountering an issue when exporting a sequence from Premiere Pro / Media Encoder.

Even though the export format is set to H.264 with Multiplexer set to MP4 and Export Audio enabled, the output renders the video correctly, but the audio is saved as a separate .aac file that cannot be played.

Here are the key details of my setup and sequence:

• Sequence Settings: 3840x2160, 50 fps, Audio: Stereo, 48000 Hz.

• Export Settings: H.264, Multiplexer: MP4, Audio: AAC 320 kbps / 48 kHz / Stereo.

• Behavior: The issue persists under both Hardware and Software Encoding. Rendering completes without explicit error codes, but the audio stream fails to multiplex into the final MP4 container.

Could you please help me identify what is causing this multiplexing failure and how to resolve it?