If voices are quieter than background noise, then it will probably not be possible to hear what is being said.
If the entire recording is too quiet, then you can use the Amplify effect.
If parts of the recording are loud and other parts are quiet, then you can even out the volume level with the AGC plug-in. See: https://forum.audacityteam.org/t/agc-automatic-gain-control/26706/1