The audio print control code can take a number to refer to a specific sfx slot. Given only the number, it plays the sfx in that slot:

-- plays sfx 12 (silent if the slot is empty)
?"\a12"

When followed by a space and additional print material, I expect it to play the given sfx as above. Instead, it plays the standard beep as if no number is present. I can further confirm that it actually stores the system beep in the numbered slot, overriding whatever sfx data was there previously.

-- beeps (expected: play sfx 12)
?"\a12 sup yall"

-- beeps
sfx(12)

When followed by note data, this works as documented: it stores the note data in the given sfx slot, plays it, and prints the rest of the string.

-- stores note data in sfx 12 and plays it
?"\a12s4Xxc1egc2egc3egc4"

-- plays sfx 12
?"\a12"

-- plays sfx 12
sfx(12)

-- beeps (expected: play sfx 12)
?"\a12 sup yall"

-- beeps
sfx(12)