you can use the Tiny Encryption Algorithm (TEA) to design your system. it implements the following operations:
�� Bitwise XOR, AND or OR
�� Addition or Subtraction modulo 2
�� Shift or rotation by a constant number of bits
TEA is not a good crypto better use AES128 or if needed XTEA, TEA has been broken already :-(. I implemented AES128 in 460 bytes including look-up table on PIC. For voice you need to use a kind of stream cipher (CFB, OFB) and not a block cipher.