what signed 16 bits little endian mean in pcm audio buffer?
Endianness refers to how multi-byte words are organized, supposing that a byte is an 8-bit word. For big-endian, ordering puts  the most significant byte first and the least significant byte last. This is the converse for  little-endians. 

An unsigned hexadecimal number 0x2317 needs  two 8-bytes


big-endian order: 0x23 0x17,
little-endian order :  0x17 0x23.