FreeClimb API supports playback of single-channel, 8 kHz, 8-bit μ-law WAV files. These files must be stored in a location that is available via URL. You can convert files to this format using an audio editor. Audacity is a free, open source option.
Import the source audio.
Adjust the project rate to 8 kHz.
If the source is recorded in more than one channel, convert it to mono.
Export the track, selecting the μ-law encoding.