It allows you to preview how Second Life voice morphing works; see this SL Wiki page for more on voice morphing. You can also subscribe to voice packs from here. Each Morph Pack subscription entitles you to 30 days of usage with each morph in the pack. Morph Pack subscriptions are stackable, so each successive subscription to the same pack adds 30 days to that pack's expiration date.
To preview a voice morph, you first need to record a sample. Click the Record button to record a brief sample of your own voice using your computer's microphone. When you are done recording, click Stop. Once you have recorded a sample, you can click any Voice Morph in the list to hear your voice as it will sound to others when that morph is active.
If you find a morph you like, you can click the Subscribe Now link. Opens the Second Life website, where you are provided with a SLurl to a load-balanced instance of Voice Island. Voice Island hosts inworld vending machines for all Morph Pack subscriptions.
The list below shows all the available voice packs, along with the expiration date of your subscription - for any to which you have subscribed.
Voice Morphs can be accessed and activated from the Nearby Voice panel, which you can open by pressing the arrow next to Speak on the bottom bar.
NOTE: Voice morphing is provided by LL, not by the Firestorm Viewer nor the Phoenix Project.