Let's start with making sure Firestorm voice is set up. Hover your mouse over the media play button, to display the media controls, then click the gear icon to jump to Sound & Media preferences. Alternately, press Ctrl-P to open the Preferences window, then click on the Sound & Media tab. Then click on Voice Settings at the top of the window.
Voice Chat should be enabled. And just below that are some voice settings:
The default speak key is your middle mouse button. Click the Set Key button to choose a new key, or the reset button to return to the default.
Clicking the Audio Device Settings button brings up, obviously enough, the audio device settings. Here you can choose from the default input and output devices based upon your particular computer and operating system configuration. The volume slider is used to adjust your microphone volume. Speak into your mic and raise or lower the volume until you can light up all the green squares, but none of the red ones.
Then click OK. Remember that these settings will unset if you crash; be sure to use Ctrl+Q or Avatar → Exit Firestorm to close Firestorm properly and better assure saving your settings.
You can tell that the voice system is working when a small white dot hovers over your head, as well as those of others who have voice enabled.
To talk, you can press your speak key (middle mouse by default) to open your mic, release it to close your mic. If you enabled toggle, then you press and release your speak key to talk, press and release again to close the mic.
On the bottom bar, if enabled, is a speak button. Click and hold it to open the mic, release to close the mic. If you enabled toggle, then clicking the speak button opens the mic, clicking again closes it. The padlock on the speak button indicates whether it is in toggle mode. A small circle will light up around it when it is enabled. When an avatar speaks, even you, green (or red) waves will emit from the speaker's white dot, from the mic icon in the nearby chat bar, from the mic for that avatar in the nearby voice panel, and, for you, the mic icon on the speak button.
The up arrow right next to the speak button brings up your Nearby Voice panel. You can change your voice to a subscribed morph, or disable morphing 1). For others, hover over their name and a i in orange circle appears. Click it, and you will access their brief info and can adjust their voice volume independent of others.
Please refer to this page to troubleshoot voice issues.