Found this, once I'd waded through the small ads in The Compleat Ankh-Morpork City Guide, and discovered the Clubs and Societies section:

The Ankh-Morport Recovering Accordion Players' Society

To lie in thrall to the muse of music can be a curse to the few as it is to the many, and the stress of playing the accordion to a chorus of "bloody well stop playing that damn thing" does take its toll after a while. Sufferers are understood and welcomed, and in some cases, with help, are encouraged to learn the banjo, thus rehabilitating them into society. Donations are always welcome.

No harmonica or squeeze-box players admitted.

The nearby panel advertisement shows a chap sat on a chair playing a piano-accordion, with a notice on the wall behind him saying DO NOT DISTURB. At his side is a woman sitting arms folded, with a facial expression best described as "seriously dischuffed, but determined to not explode .... yet".