Learn English – What does the idiom “to sound a little off” mean

It means “to sound not quite right”. Three rather literal examples plucked from the web:

“Sometimes the bass on music will sound a little off, almost distorted.”
“it sounds a little off ’cause i still haven’t tuned it”
“Starting this morning when i turned the truck on i noticed the engine sounds a little off. Kinda like one cylinder isn’t firing. ”

It can also have a slightly metaphorical use, to describe a situation that is not right: e.g., “the price they quoted sounds a little off”.

In a related vein you have “to feel a little off” meaning to feel not quite right, applying both to people and things: “I feel a little off after eating the whole tub of icecream”, “the atmosphere of the meeting felt a little off to me”.