1
general::
In excellent condition, as in Now that the brakes have been relined, the car is sound as a bell, or The surgery went well and now he's sound as a bell. This simile rests on the assumption that the bell in question is not cracked (which would make it useless). First recorded in 1565, it has survived numerous other similes (sound as a top or roach or dollar), probably owing to its pun on "sound."
American Heritage Idioms
2
general::
Phrase(s): *sound as a bell
in perfect condition or health; undamaged. (*Also: as ~.) • The doctor says that the old man’s heart’s as sound as a bell. • I thought the vase was broken when it fell, but it was sound as a bell.
McGrawhill's American Idioms And Phrasal Verbs