enable the 'use full wake lock' setting in the advanced section of the Preferences screen. Most Android devices have a "feature" in their network stack which stops it receiving broadcast packets when the screen turns off, even if an application requests a CPU, Wifi and Multicast wake lock. Basically the phone ignores the app here. Google don't seem to care about this issue (it's been around for years) and the recommended "solution" is a full wake lock (keep screen on, which is what this setting does), or stop and restart the WiFi when the screen switches off, which kinda works but (depending on your phone) can take 10 or more seconds and is a piss-poor "solution" and did actually make me quite angry.
To use voice recognition you have 3 options. The standard way is to assign a controller button (use the Assign Buttons screen for this - choose whether you want to keep the button held down while you talk - I recommend having this option *un*checked to start with). The app will by default disable the annoying Google voice recogniser bloop sound, but if you're struggling to make it work try re-enabling this - again, preferences screen -> advanced (near the bottom), *un*check the "Mute default voice recognition beep" option, and *un*check the "Play listening beep". This makes the app use Google's own voice recogniser bloop which is a more accurate reflection of when the app is ready for speech input (but sounds shit). If you're not hearing this then something's not right - the app can't access voice recognition.
If you do hear the bloop sound, speak after it's played and ask something simple like "what's my position?". Do this when the game is running and you're participating in a session.
If you don't hear a bloop sound there's not a lot I can do - the app requests access to the voice recognition service and sometimes it just doesn't work. To be honest, Android is a bit like that. Flaky... You could try forcing the app to stop then starting it up again, or even rebooting your phone, or hitting it with a hammer, or writing a strongly worded letter to Google asking them to make their APIs either work all the time, or none of the time. Working some (or even most) of the time is just idiotic.
A more reliable way of getting voice recognition to work is to enable the "Show Radio button on main screen" option. When you tap this the voice recogniser should start (again, subject to how your phone is feeling at the time and whether the Google voice recognition servers decide they like you).
The 3rd way is to enable "Listen for voice near start line" (only works if there's no button assigned to Open radio channel). This listens without any user input for a few seconds when you're 200 metres from the start line - again, you should hear the beep / bloop noise telling you it's ready.
The app does *not* listen continuously. If you had a response then something must have triggered the recogniser. It requires a decent internet connection to work properly (recognition will usually be done on some server somewhere - there's a "prefer_offline" option in the API but it turns out that this isn't a "prefer" setting, it actually means "do this offline or don't do it at all", and generally *never* works). If you have a Samsung device (or presumably some others) check that the default voice recognition setting is the Google recogniser, not some silly manufacturer one like Vlingo.
Button assignments will work with wheel buttons soon - hopefully when the next game patch is released so these are in the UDP data sent by the game.