winmm.dll
This file is not a system component: windows itself, not a download. Restore it from the source that installed it rather than downloading a replacement.
Do not download this DLL on its own
Searching for a missing DLL by name leads to sites that hand you that single file. It is one of the oldest and most effective malware delivery methods there is, and a system DLL from an unknown source is indistinguishable from a deliberately modified one. There is never a legitimate reason to obtain a runtime component that way.
What it is
The Windows multimedia API, which handles legacy audio playback, MIDI and joystick input. It ships with Windows and always has.
Where you see it: Old games and audio software. Very frequently on games that have been modded, because several mod loaders install themselves as a fake winmm.dll next to the executable.
Check the game or program folder for a winmm.dll sitting next to the .exe. If there is one, it is a mod loader or an ASI hook, not the Windows file, and that copy is what is failing: rename or delete it and the program will fall back to the real system library. If there is no such file, repair Windows with "sfc /scannow" instead of downloading anything.
Why there is nothing to download here
There is no verified download we can offer for this one, because installing a file is not the fix. The steps above are what actually resolves it. That distinction matters: the sites ranking for this file name are selling a download that would not have helped even if it were safe.