This is a pre-compiled DLL derived from the MAME Yamaha YMF262 FM synthesizer chip emulator.
The source originates from the emulator found in DOSBOX prior to the point where DOSBOX switched from this MAME emulation to using an emulation of their own making.
The DLL can be downloaded from here.
The source code can be fetched from here.
hg clone http://hg.kewl.org/pub/YMF262.DLL
The DLL can be used with Freq Monster 801, however, an example class is incorporated with the source code giving a hint about how you can use this DLL in your own application. Look in the example directory.