The Institute for Justice is challenging Philadelphia's new requirement that tour guides be licensed and take special history and geography exams. IJ seems to be exactly right on this: Such a requirement violates the First Amendment.
The government surely couldn't require that authors of history books or travel books be licensed and take exams. Nor can it require the same as to producers of travel documentary DVDs, or actors in theaters that present history/geography-related informational entertainment.
Such restrictions would be seen as unconstitutional content-based prior restraints, even if they don't purport to directly control (say, through loss of license) what is said: They bar speakers from speaking until a license is received, they are justified by the desire to prevent speech with a bad content, and they apply only to speakers who would convey a particular sort of content (history or geography in my hypotheticals).
The answer isn't any different for tours, which are likewise a form of history-/geography-related infotainment. They are speech in the quite literal sense of the word, and they are no less constitutionally protected than are books, videos, or plays.
None of this speaks to content-neutral requirements aimed at non-content-related harms, for instance requirements that theaters provide ample fire exits, or that tour operators use only licensed drivers and carry adequate liability insurance. But the Philadelphia requirements, as I mentioned, apply to speech of a particular content (tours that "provides information on the City's geography, history, historic sites, historic structures, historic objects or other places of interest"), and they are aimed at preventing allegedly misleading content. To make things even clearer, an exemption for tour companies that educate their own guides is applicable only after a content-based judgment on the city's part -- "[i]f the Department determines that the educational program and method of evaluating tour guides is equivalent to or exceeds the written examination required by this Section" -- though even if this exemption was removed, the rest of the ordinance would still impose an unconstitutional content-based prior restraint.
Just to anticipate some rebuttals: (1) Tour guide speech doesn't fit within the "commercial speech" zone of lowered protection, because that zone basically covers only commercial advertising, not speech sold for money (or else ordinary books, newspapers, movies, plays, and the like would generally be "commercial speech" so long as customers had to pay for them). (2) Nor can such licensing and testing systems be justified by general consumer protection rationales, or by a theory that sellers of speech are engaging in misleading advertising when they promote "historical tours" that teach bad history; again, consider the analogy of books, videos, and the like. (3) Licensing and testing is of course commonplace for professional-client business relationships, including ones that chiefly consist of speech, such as lawyer-client or psychotherapist-client relationships; but the premise there is that these involve personal advice aimed at a particular person's situation, and usually in high-stakes contexts -- they surely don't apply to tour guide / patron relationships any more than to author / reader relationships. (4) Naturally, the government as employer may impose various credentialing requirements on employees who talk on its behalf, but that's not what's going on here.
If the government is really worried about tourists' being misled, and the problems not being resolved through market pressure (tours, after all, don't like online criticisms, or people telling their hotels' concierges what a bum recommendation the concierge had given), the government can easily set up an optional "Seal of Approval" system; and tours that get the Seal will surely promote it as a sign of superiority over others that don't have the Seal. But mandatory content-based licensing and examination requirements for speakers are unconstitutional, and a very bad precedent that could easily be extended to other classes of speakers as well.