Alcohol's effect on tinnitus varies widely. Some people find a glass of wine briefly quiets the ringing — vasodilation and sedation both contribute. Others find tinnitus is markedly worse the morning after, and worse still after a heavy night. Both experiences are real, and they happen in the same person at different doses.
A few honest mechanisms. Low to moderate alcohol mildly suppresses central nervous system activity, which can lower tinnitus perception in the short term. But alcohol fragments sleep, especially in the second half of the night, and under-slept brains are louder brains. It is also mildly ototoxic at high doses, which is one reason chronic heavy drinking is linked with hearing problems. And dehydration on its own raises blood viscosity, which can worsen any vascular contribution to tinnitus.
A reasonable rule: light, occasional drinks are fine for most people with tinnitus. Heavy or daily drinking tends to make things worse over weeks, even if any individual evening feels better at the time.