With few exceptions, buildings (as well as employers, public entities and transportation) have to meet the Americans with Disabilities Act (ADA) which was passed and signed into law in 1990 and effective in 1992. It was amended in 2008 to include broader protections for people with disabilities, which is defined as: "a physical or mental impairment that substantially limits a major life activity."
Title III deals with Public Accommodations and Commercial Facilities, basically anyplace that is open to the public which would definitely include music venues of all sizes.
"Under Title III, no individual may be discriminated against on the basis of disability with regards to the full and equal enjoyment of the goods, services, facilities, or accommodations of any place of public accommodation by any person who owns, leases (or leases to), or operates a place of public accommodation."
Title III applies to "new construction" which also includes any remodels or renovation done to a building constructed prior to 1992. It also requires pre-1992 buildings to comply with the ADA by "removing architectural barriers" that would preclude anyone with a disability from accessing the building. The exception is that the removing of barriers is to be "easily accomplished" by the building owners, and historic buildings are only required to do it to the "maximum extent feasible."
I don't know about other cities, but living in the Denver-Metro area I can tell you that here there are very few places that aren't wheelchair accessible. In some music/entertainment/concert venues you will get a better seat than most people, in others (usually the more historic venues) you are much further back from the stage. It all depends on the venue. But I can't remember a place that doesn't meet the requirements of the ADA. I know there are places that have an exemption, but I think most places that are open to the public have done all they are required to and more, in many cases.
In addition to the accessibility to the building, the ADA also requires that there are designated disabled parking spots (you have to have a placard to legally park there), as well as restroom facilities that are accessible to the disabled. Those guidelines are specifically for wheelchair access, with at least one space for a toilet wide enough to accommodate a wheelchair and with hand rails. The lavatories (sinks) also have to be a certain height, as well as drinking fountains and things like that. (I work for a mechanical and plumbing engineering firm, so I see the submittals for those fixtures with every job we do.)