A library, especially a meeting room or media listening room, where playing your recording won’t disturb anyone else.
Many outdoor public places that are remote enough in time (particularly, at night or early in the morning) or space from cars, machinery or activity.
Bottom floor of an underground parking lot.
Indoor phone booths, in places that still have some of those.
Currently-not-being-used-much parts of publically-accessible municipal buildings.
Currently-not-being-used-much parts of a college campus.
Currently-not-being-used-much parts of office buildings.
Currently-not-being-used-much parts of medical/hospital buildings.
Currently-not-being-used-much parts of shopping malls or department stores.
A beach that is behind a cliff in the direction of the urban area.
At the bottom of a ravine in a park. Or just far enough away from roads in the wooded part of a park.