So here's a thought: since Sound Manager 3.0 patches out the previous Sound Manager, and System 7.5 integrates it into the system...
...could we take the system resources from 7.1 for Sound Manager 2.x, toss them in an extension, label it version 4.0, and have it patch out 3.0 in System 7.5? That way, for older software, the older Sound Manager would be used, and you could disable the extension to get Sound Manager 3 handling back for software that needs it.