When working with Android SoundPool you can alter the playback rate with the following API and adjusting the rate:
SoundPool play(int soundID, float leftVolume, float rightVolume, int priority, int loop, float rate)
My question is how to do so within the Android oboe library. I know you can set the frequency to a AudioStreamBuilder, but once the stream has been opened, can you then change the rate on the fly?
Resampling is not supported in Oboe. It might be added in future, however, it's more likely to be added in a supporting library or in a sample.
If you want to implement this you could use libresample from FFmpeg (here's how to add FFmpeg to Oboe).