MIPS MIDI Player: problems with playback and skip - mips

Question

I have a couple of questions about my group project. The first question is regarding playback of a MIDI file. Right now our program can playback a type 0 file with about 75% accuracy (if you can even gauge that). Basically, it get "most" of the song right (or really close). It's clearly not playing every sound and some of the ones it does play sound distorted. So if it's playing most of the notes correctly, what could cause it to miss or distort the others?
The second question is concerned with our interrupt handler. I'm trying to adjust the pointer so the program will skip X bytes forward/backward, but if I try the interrupt while the program is running it stalls and locks the program up. If I run through the interrupt step-by-step it won't stall. It seems like I'm either halfway there or totally off the mark (this was working on an earlier version, but probably b/c the playback function wasn't written correctly). If I had to guess I'd say the problem has something to do with the WITH_STATUS and WITHOUT_STATUS functions. Is that right?
Thank you in advance for any help.
PLAY_MIDI:
addi $s5, $0, 0 # temporaries to control speed and volume
addi $t6, $0, 100
addi $t9, $0, 0
addi $sp, $sp, -4 # add a byte to the stack pointer and store the return address there
sw $ra, 0($sp)
addi $t4, $0, 3
LOOP:
lh $t0, DAT_Format
bnez $t0, NO_SUPPORTED
addi $s3, $s3, 1
addi $s1, $s1, -1
lbu $t9, 0($s3) # look for status byte
blez $s1, OFF_LOOP
bltu $t9, 0x80, LOOP
bgeu $t9, 0xa0, LOOP
WITH_STATUS:
lbu $t9, 0($s3)
lb $t6, -1($s3) #sleep time
lb $a1, 1($s3) # load note
lb $a2, 2($s3) # load volume
addi $s3, $s3, 4
addi $s1, $s1, -4
move $s2, $s1 # *** copy $s1 to $s2 for pause/play
j SLEEP
WITHOUT_STATUS:
lb $t6, -1($s3) #sleep time
lb $a1, 0($s3) # load note
lb $a2, 1($s3) # load volume
addi $s3, $s3, 3 #increase stack
addi $s1, $s1, -3
move $s2, $s1 # ** copy $s1 to $s2 for pause/play
SLEEP:
mul $t6, $t6, $t4
li $v0, 32 # sleep until the next note should be played
addu $a0, $0, $t6 # sleep for $t6 ms
syscall
and $a0, $t9,0x0F # load channel
and $t9,$t9,0x000000F0 # load Command
######################################################
## copy $s1 to $s2 for use differentiating between ##
beq $t9, 0x90, PLAY_NOTE_ON ## stop and pause ##
beq $t9, 0x80, PLAY_NOTE_OFF ## $s2 will hold last position of $s1 ##
bgeu $t9, 0xa0, LOOP ######################################################
AFTER_PLAY:
blez $s1, OFF_LOOP
lbu $t0, 0($s3)
bgeu $t0, 0x80, WITH_STATUS
j WITHOUT_STATUS
OFF_LOOP:
bne $s1, $s2, PAUSE_LOOP # get the return address from the stack and decrease the pointer
lw $ra, 0($sp)
addi $sp, $sp, 4
jr $ra # jump to the address we just loaded
PLAY_NOTE_ON:
li $v0, 60 # add a byte to the stack pointer and store the return address there
syscall
j AFTER_PLAY
PLAY_NOTE_OFF:
li $v0, 61
syscall
j AFTER_PLAY
NO_SUPPORTED:
j EXIT
#### Below is part of the interrupt handler ####
SKIP_FORWARD: addi $s3, $s3, 1 # by adding to $s3 program will move pointer ahead
la $a0, skip_forward # changed from $s3 to $s1 ***
li $v0, 4
syscall
jr $ra
SKIP_BACKWARD: addi $s3, $s3, -1 # by subtracting from $s3 program will move pointer backwards
la $a0, skip_backward # changed $s3 to $s1
li $v0, 4
syscall
jr $ra