Strange Sound from a 100V Line PA Amp

[crc-ict]

We have an Adastra 100V line PA amp here connected to a couple of sets of outdoor speakers.

 

One of the sets of speakers here is on the field, and when they are the only speakers switched in, the speakers output can't be heard from the amp position.

 

However, I have noticed today they the music appears to be emitted from the circuitry of the amplifier itself, similar to the sound you get from the stylus of a turntable?? It's not entirely recognisable as to what song is playing, but it is definitely a music sound - the rhythm/beat is recognisable.

 

This seems strange to me - does anyone know if it is normal, and why it does it?

 

Puzzled...