If you're talking about making a sound:
* Loudly
* Aloud
* Audibly
* Noisily
If you're talking about speaking:
* Verbally
* Orally
* Vocally
* Out loud
If you're talking about doing something without drawing attention:
* Openly
* Publicly
* Obviously
* Noticeably
Choose the antonym that best fits the specific situation you're describing.