If you go into a map, slow it down to any speed (75/50/25) unplug your headphones, then re-plug them in, the song has some sort of daycore-sounding filter on it.
Which version are you editing the map? I tried on stable and it doesn't happen, but in lazer it always happen. Perhaps it's intentional since the audio is stretched rather than resampled to slower BPM, hence the low pitch.