Short beeping noise for low HP

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+When your Pokémon's HP is in the red, the battle music is replaced by a continuous beeping noise. This can be quite annoying. If you want a brief alert that your HP is low, just follow this tutorial.
+
+Edit [audio/engine.asm](../blob/master/audio/engine.asm):
+
+```diff
+ UpdateChannels:
+ 	...
+
+ .Channel1:
+ 	ld a, [wLowHealthAlarm]
++	cp $ff
++	jr z, .Channel5
+ 	bit DANGER_ON_F, a
+ 	ret nz
+ .Channel5:
+ 	...
+```
+
+```diff
+ PlayDanger:
+ 	ld a, [wLowHealthAlarm]
+ 	bit DANGER_ON_F, a
+ 	ret z
++	cp $ff
++	ret z
+
+-	; Don't do anything if SFX is being played
+-	and $ff ^ (1 << DANGER_ON_F)
+ 	ld d, a
+ 	call _CheckSFX
+ 	jr c, .increment
++	ld a, d
+
+ 	; Play the high tone
+-	and a
+-	jr z, .begin
++	and $1f
++	ld hl, DangerSoundHigh
++	jr z, .applychannel
+
+ 	; Play the low tone
+ 	cp 16
++	jr nz, .increment
+-	jr z, .halfway
+-
+-	jr .increment
+-
+-.halfway
+ 	ld hl, DangerSoundLow
+-	jr .applychannel
+-
+-.begin
+-	ld hl, DangerSoundHigh
+
+ .applychannel
+ 	xor a
+ 	ldh [rNR10], a
+ 	ld a, [hli]
+ 	ldh [rNR11], a
+ 	ld a, [hli]
+ 	ldh [rNR12], a
+ 	ld a, [hli]
+ 	ldh [rNR13], a
+ 	ld a, [hli]
+ 	ldh [rNR14], a
+
+ .increment
+ 	ld a, d
++	and $e0
++	ld e, a
++	ld a, d
++	and $1f
+ 	inc a
+ 	cp 30 ; Ending frame
+ 	jr c, .noreset
+-	xor a
++	add 2
+ .noreset
+-	; Make sure the danger sound is kept on
+-	or 1 << DANGER_ON_F
++	add e
++	jr nz, .load
++	dec a
++.load
+ 	ld [wLowHealthAlarm], a
+
+ 	; Enable channel 1 if it's off
+ 	ld a, [wSoundOutput]
+ 	and $11
+ 	ret nz
+ 	ld a, [wSoundOutput]
+ 	or $11
+ 	ld [wSoundOutput], a
+ 	ret
+
+ DangerSoundHigh:
+ 	db $80 ; duty 50%
+ 	db $e2 ; volume 14, envelope decrease sweep 2
+ 	db $50 ; frequency: $750
+ 	db $87 ; restart sound
+
+ DangerSoundLow:
+ 	db $80 ; duty 50%
+ 	db $e2 ; volume 14, envelope decrease sweep 2
+ 	db $ee ; frequency: $6ee
+ 	db $86 ; restart sound
+```
+
+TODO: Explain changes.
+
+With this short edit, the noise will stop after just four beeps. It plays when your HP first turns red, and when you send out a Pokémon with low HP.