The basis of an audio mixer is an inverting summing circuit below. For real audio mixers, a single-supply voltage is seldom used.
To increase dynamic range, the designer will often push an op amp up to, and sometimes beyond it’s recommended voltage rails. Noninverting summing circuits are not recommended, but it’s possible to use. The source impedance becomes one of the gain calculation part. For minimum error due to input bias current,
Vout= – R2(Vin1/R1 + Vin2/R2 +Vin3/R3)= R1A||R1B||R1C||R2.
[Source: Texas Instruments Application Note]