Ciao paddlesteamer,
the easiest way to handle a signed value is to use an signed variable!
You are using unsigned, so it became everything complicated.
Try this sample code, it implement a very simple echo:

#define ECHO_LEN (1024*32)
int16_t echoBufR[ECHO_LEN];
int16_t echoBufL[ECHO_LEN];
uint16 ...