My doc said Tinnitus was a result of nerve damage.............
My tinnitus went away when I started taking BP medication.
B vitamins help heal nerve damage.
He explained that what happens is that when hearing loss occurs (in the ears themselves) the special part of the brain in charge of hearing gets confused by the loss of "signal strength" (my words) and faced with that loss it "turns up the volume" which causes the ringing/hissing you hear.
Of course he was giving me the simplified "civilian" version of the explanation...the medical version,given that it involves the brain,is probably 575 pages long (I worked for years in health care and know that many of those medical textbooks weigh 25 pounds!).