There are 2 messages to MCI window in sound.nsi script
SendMessage $R0 0x0806 0 0
SendMessage $R0 0x0472 0 1
MSDN:
#define MCI_PLAY 0x0806
#define MCIWNDM_SETREPEAT (WM_USER + 114)
#define WM_USER 0x0400
Second message must set repeat state, but I could not reproduce this now. I'll try to undestand problem tonight.
Edit:
#define MCIWNDM_SETREPEAT (WM_USER + 14)
What number is correct? +14 not works too.