Smartphones

[sonar]

If you have a Samsung smartphone:

Settings -> Axyliary options -> Support for the hearing impaired -> Amplifying ambient sounds.

Then you can hear in the earphones what the phone's microphone detects.
(Wired or bluetooth. )

[kris80]

An alternative is the app "mic speaker" which adds some filtering options (noise suppression, echo cancellation) and configurable amplifiers for both recording and playout