"I hope I am clear"

No, you are not, but at least you try.

1) please do not start a new thread whit your answer.

2) Let me try to visiualise what you are trying to do.

a) you have one '51 connected to an ADPCM codec.
b) you have another '51 which is connected to a PC.
c) you want to transfer data from a to b.
d) you do not want to transfer data from b to a.
e) you have no more uCs in the system.

Please verify the above and answer:
aa) how fast do you need to transfer data from a to b (bytes/sec).
bb) how far are a and b from another.

Have fun,

Erik