Unlicensed operation on the AM and FM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules. On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR (Code of Federal Regulations) Section 15.239, and the July 24, 1991 Public Notice (still in effect). On the AM broadcast band, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221. These devices must accept any interference caused by any other operation, which may further limit the effective service range.
<https://www.fcc.gov/media/radio/low-power-radio-general-info...>
More on the regs: <https://www.fcc.gov/general/oet-bulletins-line#63>
Another option, addressed at the first link, is "carrier current" broadcasts, which transmit through (building) power lines.
There is also the option of a (licensed) low-power FM transmitter (LPFM), "available to noncommercial educational entities and public safety/travelers' information entities, but not individuals or commercial operations".
LPFM is not to be confused with microbroadcasting, though I'm not clear on just what that entails myself. I'm vaguely aware of some efforts to enable lawful very-low-power broadcasts at the community level, though that's had strong pushback from traditional broadcasters, including NPR in the US.
There are some Part 15 compliant transmitters and information listed here:
<https://www.hobbybroadcaster.net/resources/Part-15-transmitt...>