I've tried a few different time stretch tools and they all yield the exact same results of distortion and no high frequencies. How do you slow down samples without making them sound terrible?
REAPER has multiple different time-stretch algorithms that can be used, each with their own sound and strengths. Try experimenting with those. Additionally, if you are losing high-frequency content, consider adding a harmonic exciter to the stretched sample to add more "shine" back to it
Time stretch tools can't produce (or interpolate) frequencies that aren't there. If you are planning to time stretch a signal, sample with 96kHz at least. Assuming that your microphone and A/D converters play along, that should give you signal content (even if its quality degrades in the inaudible ranges) to somewhat above 30kHz. When time-stretching, this gives some meaningful content to bring down into the audible range rather than the dead silence above the cutoff frequency that otherwise has to be there.