Here’s a simple Mic mixer that can be Powered with 5V. There is currently 3 channels on the circuit but you can add more if needed.
About the Circuit
The circuit is made up with BJT common emitter configuration. OPAMPS that can operate at low voltage are hard to find at a local electronic stores. The very common TL072 opamps can still operate at 5V at a very limited output voltage swing.
The circuit provides ~29dB gain, you can reduce that gain by increasing the value of the emitter resistors(R8,R9, and R11).
R3 and C1 is a filter for the power supply. C1 can be increased to reduce the noise if your power supply is noisy. R3 can also be increase up to around 100 ohms for better filtering.
You can connect this circuit to the USB audio interface circuit in this post so you can have this connected to you PC for recording.
On the simulation below I used waveform generator with sinewave output at 100mV at frequencies 100Hz, 200Hz and 500Hz respectively. The volume setting is set at 30%, 10% and 10% respectively.