If "overheard" means to hear something unintentionally:
* Ignored: This implies a deliberate choice not to listen.
* Missed: This suggests that the sound was not perceived at all.
* Didn't notice: This indicates a lack of awareness of the sound.
If "overheard" means to hear something secretly:
* Witnessed: This implies a direct and intentional observation.
* Observed: This also suggests a conscious act of seeing or hearing.
* Heard directly: This emphasizes the direct and intended nature of the listening.
If "overheard" means to hear something from a distance:
* Listened to: This suggests a direct and intentional act of hearing.
* Heard close-up: This emphasizes the proximity of the listener to the source of the sound.
The best antonym will depend on the specific context in which "overheard" is used.