The only way I know of at the moment would be to route it through multiple AudioFaders before it reaches the end destination, as that will stack the volume increase.
As an example, if you have it as AudioDeviceInput → AudioFader → AudioEmitter at the moment, it could become AudioDeviceInput → AudioFader – AudioFader2 → AudioEmitter.
Or (I haven’t tested this one yet), but the AudioFaders might be able to be added in the chain after the AudioListener → AudioFader1 → AudioFader2 → AudioDeviceOutput if you’re trying to make a type of change where players can customize all of the incoming audio they’re hearing from other players.
It’s not the most elegant solution but it could be a viable option in the meantime. If you think the max volume for the AudioFader should be increased, consider making a feature request for it or suggesting it in the Audio API announcement thread since it’s in beta at the moment so features are still subject to change.
I personally have not encountered issues with the voice chat audio being too quiet with the new Audio API, so I would recommend:
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Making sure that both the in-game volume and the volume mixer for the Roblox client (via the sound settings for your OS) is not set too low.
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Making sure that the microphone input volume is not too low, too (if any of the people you were testing that with are willing to check that; whether through the microphone itself or via built-in OS sound settings).