Firstly, you should only expect 10-bit performance from the AVR's ADC at 10k samples per second, barely a high enough rate for telephone quality voice. And that's mono, not stereo.
Second, the AVR family doesn't have DACs built-in so you will have to add your own. If you have a lot of AVR pins, you could put an R-2R on them, or you could use the PWM.
Also, if you don't already have your ATMega324-20pu parts, beware they are not recommended for new designs. If you're just playing this isn't a problem. For a commercial project is pays to use a part with a future supply.
How would you go about programming this? Well, start on page 240 of the ATMega164P/324P/644P data sheet.