How to use a 8051 and an ADC0808 to generate any sound (may be a song or voice even if it is in telephone quality) that the microcontroler gets from the uart.
Correct me if I'm wrong, but you are going to transfer the sound to the 8051 using the serial port. Then you are going to play the sound using the ADC0808?
The ADC0808 is quite properly named. It is an 8-channel, 8-bit analog-to-digital converter. It is used to record sound, not play sound.
You can hook up a microphone using an Op-Amp to take in sound that way - not generate the sound.
If you send the sound to the 8051 using the UART, you will not need any ADC0808.
The 8051 is able to generate sound. You may produce sound ising PWM - pulse width modulation - that you send to a low-pass filter and then to some form of amplifier before driving a loudspeaker. For low volumes, you may also skip the amplifier, but should then select a loudspeaker with quite high impedance.