Y:
Only 931 hits on Google for your topic. I found 3 noteworthies but
prolly more.
Looks like this treatment may have some success for T not caused by
noise damage.

-Pubmed Abstract
Tinnitus suppression by electrical promontory stimulation (EPS) in
patients with sensorineural hearing loss.
CONCLUSION: In our opinion electrical stimulation by using positive DC
changes the spontaneous activity of cochlear nerve fibres. According to
our results it is suggested that the mechanism of beneficial effects is
due to increased microcirculation in part of the auditory pathways.
Poorer results in patients with noise induced tinnitus could be
explained by greater damage of the cochlea outer hair cells. In our
opinion EPS could be a method of treatment for persistent tinnitus in
cases which fail to respond to other methods.

-This guy (surgeon) tried it, says it didnt help:
http://www.tinn.com/clinics.html

Transcranial Electrostimulation (TES) Study:
-http://www.ifess.org/cdrom_target/ifess99/Free%20Paper%20Session%207/lebedev.pdf
"Sensorineural hearing loss (SNHL) based on impairment of neural
structures of inner ear was effectively treated by TES (group D). The
first effect in 50% of P with chronic SNHL (who were ineffectivly
treated by medications by the years) was the reduction or abolishment
of subjective tinnitus."

Gotta love google.  I'm not sure if this site is product oriented, but
it looks like a dentist website.   See the study ""Transcutaneous
Electrical Stimulation for Tinnitus."" on this  page half way down.
http://www.reiddds.com/products/asresea2.html.  Seeminly more positive
results for TES.