Parameters for all media converters are:

url - Input file to convert, supports direct links to media files, youtube urls or tiktok urls.
quality - Which quality you want to play back at, low (optimised towards modem/isdn), mid (mobile broadband), high (DSL)

Example command for playing back a youtube video as an h264 mp4 at mid quality with ffplay:

curl "http://muellers-software.org/live-encode/video/mp4-h264.php?quality=mid&url=https://youtu.be/dQw4w9WgXcQ" | ffplay -

Example command for playing the same video as a wave file on low quality into the sound sampler device:
curl "http://muellers-software.org/live-encode/audio/wav.php?quality=low&url=https://youtu.be/dQw4w9WgXcQ" > /dev/dsp