I can't play Sorenson 3 video on Quicktime X

Interesting topic. See if the following helps.

Format: Sorenson Video 3 Pro, 848 x 480

AAC, 44100 Hz, Mono

Believe the Sorenson Video 3 Pro codec was a paid multi-pass VRB upgrade to the Apple Standard Sorenson/Sorenson 3 video codec package. Assume the standard package supports Sorenson Video 3 Pro playback but cannot test this hypothesis as I only have the standard codec currently active and no Sorenson Video 3 Pro sample files available for testing. If so, then...

I already tried enabling legacy codecs on the command line, but I still can't see the video.

Under Mavericks and Yosemite, legacy codecs are, by default, available for passive (playback) support. I.e., it is only necessary to re-enable legacy video codecs is you plan to use them actively for export in QT 7 based apps or similar third-party apps like MEG Streamclip. Unfortunately, the "Modernizer" routine for QT X will only pass the data to the converter if it is programmed to do so. If you have the QT 7 player installed on your system, suggest you try playing the file in that media player. If the file plays, then you can either use this player for playback. In addition, if you have the QT 7 Player keyed for "Pro" use or have the MPEG Streamclip app installed on your system, then you can also convert such files to H.264/AAC for compatibility with all current QT media players. On the other hand. if the files are not playable, then it would seem to indicate that the standard Sorenson Video 3 package does not support Sorenson Video 3 Pro playback.

Also of interest to me was the fact that the FFmpeg codec package also supports Sorenson Video/Sorenson Video 3 content. Unfortunately, nothing was stated regarding "Pro" variation playback support in the articles I read. Basically this means that apps, such as VLC, HandBrake, etc, may or may not also support the playback and/or conversion of your Sorenson Video 3 Content. Would, therefore, also recommend you also test your files in these or similar apps. My personal preference for conversion would be the latest version of HandBrake.

Thanks for your message. I installed Quicktime 7, but I still get the same thing, a black screen and only audio.

I also tested with VLC and Handbrake with the same result.

Any other ideas?

Unfortunately, no. It appears the "Pro" variant requires the same "Pro" codec for playback. I created an MOV file using standard Sorenson Video 3 video with AAC audio and it was compatible with all apps mentioned. I even converted it back to an MP4 (H.264/AAC) file that was then compatible with the QT X v10.4 player. Other than possibly purchasing from SorensonMedia ($$$), I think you may be out of luck at this time.