Here are two pesukim. The first is from Vayikra 19:13. In Parshas Kedoshim the Torah writes: "Lo tashok et re'acha v'lo tigzol, lo talin peulat sachir itcha ad boker." The second is from Devarim 24:15. In Parshas Ki Teizei, the Torah writes: "B'yomo titen scharo, v'lo savo alav hashemesh, ki ani hu v'eilav hu noseh es nafsho, v'lo yikro alecha el Ad-noi v'haya vicha cheit."
Halacha mandates that day laborers must be paid at the end of the workday and night workers must be paid at the end of their shift. It seems clear to me that freelance musicians playing one-off club dates fall into this category, yet it is common practice for offices to wait weeks, or in some cases a month or more, before sending out payment. I believe that this practice is anti-halachink. In the case of smaller bands, therefore, musicians should be paid at the conclusion of a gig. The band owner or bandleader should bring the musician's checks to the gig and pay them upon its conclusion
In the case of larger bands, where there are many musicians perfrming at numerous events at any given time, from an organizational standpoint, it's not practical to cut the checks in advance, as there are frequently last minute additions or substitutions of musicians. In these cases, I think that there is an implicit understanding between the musicians and the office that the payment will be somewhat delayed. Even in those cases though, the bands should cut the checks as soon as possible. I'd favor a weekly bookkeeping date, but think that a bi-weekly check mailing is the outer limit.
Over the years, I've freelanced for many bands/offices who delayed payment for over a month. I've been fortunate enough to never have been in a financial situation where I was living from gig paycheck to paycheck, but there are many musicians who need each gig's earnings ASAP just to make ends meet. I think that an increased sensitivity to this Mitzvah D'oraita is warranted among the JM community.