Pygame: Noise volume changes for unknown reason

Question

I'm creating an experiment where each trial the noise increases. I created the noise using Audacity tone generator. It's a sine noise with 300frequency and 0.5amp. I've fiddled around with a bunch of different noise types but in every few trials, the noise still decreases despite the print(sound.get_volume()) saying that the noise volume is always increasing. The overall code is very long but here are the parts that involve sound:

sound=pygame.mixer.Sound('sine1hour.wav')
soundPlay = True
def set_volume(initial_v):
    if index_for_volume > 0:
        initial_v = initial_v + sound_increase_per_trial * 
index_for_volume
    return initial_v

    while pygame.time.get_ticks() < trial_stop:
        for event in pygame.event.get():
            if event.type == pygame.QUIT or (
                    event.type == pygame.KEYDOWN and event.key == 
escapeKey):
                stop = True
            if event.type == pygame.KEYDOWN and event.key == resetKey:
                reset = True # if shift key has been pressed
                print('resetted')
        sound.set_volume(set_volume(initial))
        sound.play(-1)
        clock.tick()

sound_increase_per_trial = (1 - 0.1)/40

for p in range(num_practice):
    stop = PracticedoTrial(recordData, p, sound.get_volume())
    if stop:
        break

if index_for_volume < 0: 
    doTrial(recordData, trial, sound.get_volume())

print(sound.get_volume())