I test FFplay while downloading media from streams with FFmpeg. When it's just audio (especially radio stations), in FFplay, rdft of -showmode is shown by default. The other -showmode options are video (black screen) and waves (the white waveforms that are generated by default but can be customized in FFmpeg when using the -showwaves and -showwavespic parameters).

[Attachment 67487 - Click to enlarge]

This is the code I'm currently using to download the audio from a radio.

ffmpeg -i URL -c copy OUTPUT.MKA -f matroska - | ffplay - -showmode waves
This is a variant of the same code but using pipe parameter.

ffmpeg -i URL -c copy OUTPUT.MKA -f matroska pipe:1| ffplay -i pipe:0 -showmode waves
Could FFplay visualization be changed while playing audio only? I'm sure it's possible to do something in FFmpeg to pipe it into FFplay but I have no idea how to do it.