To assume that a mural could be construed as "commercial speech" is a stretch imo. When I hear a muralist ask if they can paint some musicians on the side of a wall, the answer that comes to my mind is ABSOLUTELY. There are some exceptions like copying someone's elses work like a photograph or implying hat someone (an artist) is endorsing something is a nono, but outside those few exceptions, OF COURSE AN ARTIST CAN PAINT MUSICIANS, this is a no brainier.
The fact that the work is going on a wall is moot, it could be a painting, a sculpture, an abstract expression, but what is clearly recognized is that the act of creating artwork is considered a first amendment right that trumps copyright laws, privacy laws, and publicity laws (with exceptions of course). An artist's mural that is being used on a side of a business to attract customers and express an opinion on music and musicians is no different than a buisness owner promoting their business and products. Is an owner standing outside their music business telling people who walk by that he has wonderful music from "this artist", "that artist" or loves "this performer" or "that performer" considered an expression of "commercial speech", or violating publicity rights? The obvious answer is hell no, so why is there even a question about an artist who expresses their opinion through their choice of medium/speech?
Every case Ive seen dealing with art and artists consider the work a first amendment right and considered an expression (speech), weather the viewer understand a meaning or not. "Commercial art" typically deals with artwork that becomes "numerous", like unlimited prints or even too many prints and posters, or an ad in some kind of publication. Creating merchandise with artwork like mugs or t-shirts. For example an artist created a drawing of the three stooges and created tons of merchandise with it (t-shirts, mugs, posters), the courts found that everything he did was a violation of publicity rights except for that first original print which was protected by the first amendment and which could be sold profited from or even promoted.