Download of Max/MSP 5.1 for Mac was available from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. This software for Mac OS X was originally produced by Cycling'74. The following version: 5.1 is the most frequently downloaded one by the program users. The software is categorized as Developer Tools. We recommend checking the downloaded files with any free antivirus.
From the developer:
Interactive visual programming environment for music, audio, and media.
In use for over twenty years by performers, composers, artists, scientists, teachers, and students, Max is the way to make your computer do things that reflect your individual ideas and dreams.
You may want to check out more software for Mac, such as SoundFilesMerger, which might be related to Max/MSP.