TumbledownGoofs
Factual errors
The portable 4-track machine does not have built-in speakers and would have needed to be hooked up to the sound system. As it was stored in a canvas bag and didn't have any audio cables connected, it should not have been able to produce any sound.
Miscellaneous
The mixing equipment in the late husband's shed, while passable for a band, was far more elaborate than would have been necessary for the music heard in the film, as (except for the "lost" song, which had a touch of piano) it was just solo acoustic guitar and solo voice.