Welcome to Android Central! Which phone? It could still be the primary mic (i.e., the one at the bottom of the phone). The voice recorder typically uses the secondary mic (at the top of the phone). Since speakerphone also uses the secondary mic, you can see how well people can hear you on speakerphone. Or you could record a video and then listen to it with headphones, to see if one channel is low or absent.
If it seems to be a problem with the primary mic, then make sure the opening isn't blocked (by a case, for example), or clogged by debris (like lint).