Here are some other words that could work depending on the context:
* Unheard: This implies that the sound was not noticed, not necessarily that it was impossible to hear.
* Silent: This refers to the absence of sound, not necessarily the inability to hear it.
* Deafening: This describes a sound that is so loud it makes hearing other sounds impossible.
* Soundless: This describes a situation where there is no sound at all.
Let me know if you have more context and I can help you choose the most appropriate word!