Here is another Op-amp based schematic. This is a copy/psate from the Atmel AVR335 "AVR Digital Sound Recorder" Application note (Page 7).

The upper circuit amplifies the input audio signal then this would be given to the 8 bit ADC for a 8 Khz sampling.
The lower one converts the PWM OUTPUT of the MCU to the real audio signal.
I am planing to use this in a small portable battery powered device.The rest of the circuit is powered by a step up at 3.3V (If required i could supply a negative voltage like -3.3V). Since in this voltage LM324 would not output more than .1mW I would like to know what else could i use here ? e.g. what Op-Amps or maybe a discrete Class D amp do you suggest ?
There are neat Class D amplifiers in 8 pin capable of driving up to 3W but since they are expensive and not available where I live, I would like to stick to discrete parts or use the conventional OP-Amps.
I would like to have 50mW output (enough for a little loudspeaker like the ones in cellphones).

Thanks in Advance

Farshid