The RC-5's INPUT jacks (A/MONO, B) are not designed to accept microphone-level signals.
The nominal input level is -20 dBu, intended for instruments such as guitars, basses, or effect units. Connecting a microphone directly will usually result in very low volume.
To use a microphone with the RC-5, connect a device that boosts the microphone signal to line level, such as a mixer, microphone preamp, or microphone effects unit, between the microphone and the RC-5.
When using a mixer or microphone preamp, use a microphone that is compatible with that device's input specifications.