From the Texas Penal Code, Section 1:
(8) "Bodily injury" means physical pain, illness, or any impairment of physical condition.
A maxed-out bullhorn from within arm's reach would certainly cause physical pain, and if it leaves your ears ringing, your physical condition has been impaired.
(46) "Serious bodily injury" means bodily injury that creates a substantial risk of death or that causes death, serious permanent disfigurement, or protracted loss or impairment of the function of any bodily member or organ.
This one would hinge on what constitutes "protracted impairment." I suspect that if you were still suffering provable hearing loss by the time the trial finally rolled around, it wouldn't take Johnny Cochrane to convince the court that that would qualify.