If the musician puts up a sign that says "Permission to use, copy, modify, and distribute this performance for any purpose with or without fee is hereby granted

If the musician puts up a sign that says "Permission to use, copy, modify, and distribute this performance for any purpose with or without fee is hereby granted", then yes, it would be ethical to record/stream it for profit - they've explicitly told you it's okay. Substituting "software" for "performance", that's what the curl license says. https://curl.se/docs/copyright.html https://curl.se/docs/copyright.html