Rogue Amoeba has released their new Mac app SoundSource:
From SoundSource’s menu bar icon, you can instantly configure the audio devices your Mac uses for Input, Output and Sound Effects. In seconds, you can adjust the volume for each of your audio devices or switch between connected devices. SoundSource can also enable the soft play-thru of audio from input devices. Use the Play-Thru window to monitor any connected input, such as a microphone, right through your headphones or other output.