Why the limit is length, not file size
To play a sound backwards, ToolGPT has to hold the whole decoded track in memory before it can write the first second of the result. So for this tool, the length of the recording matters more than the size of the file.
The practical limit is about 45 minutes of CD-quality stereo audio, and roughly twice that for mono. If a file is too long, ToolGPT tells you before anything runs, using the file's own numbers. Most songs and voice memos are well within the limit.