Avidemux Cannot Use That File As Audio Track Exclusive -

If the audio file's sample rate is invalid (e.g., 44.1 kHz vs 48 kHz) or if the audio file is significantly shorter or longer than the video, Avidemux may fail to load it. Solutions: Fixing the Audio Track Error

Avidemux historically struggles with VBR MP3 files. It much prefers Constant Bit Rate (CBR) files. avidemux cannot use that file as audio track