Hey there.
I think I’ve found a bug in the latest version of Soundbox V1.2 regarding the ping pong sample loop.
As soon as the play bar reaches the end of the sample audio is broken and silenced. When using standard loop the error doesn’t occur.
I’ve created a small video about this issue: https://youtu.be/mlBjm_vP_xQ
Sorry about this. We’ve a new version available (1.2.1) which solves the issue. You can grab the update from the Soundbox page here: https://audiomodern.com/soundbox/
Thanks for documenting this so clearly. It’s a very specific glitch with the playback engine’s loop logic, so having the video example is great for comparison even after the patch.