I'm building an application in PHP and I was wondering if I can use the Audio chat service from Google talk into my web application?
Yes you can. Google identify Google talk as "[An open IM] service so that you can hook your own client applications into the Google Talk service, or you can connect (federate) your service with ours. "
To run a Voice Chat into your application you can go through what Google is explain on:
Making a Call
"The high-level object that manages the important actions in a voice call is called (appropriately) Call. A Call object manages any number of peer-to-peer Session objects, each representing one peer-to-peer connection. The Call object is the top level object to make calls, accept or reject incoming calls, monitor the status of the call, and performs other high level actions on call connections. CallClient wraps all required steps for making a call in its MakeCallTo method."
Receiving a Call:
"An incoming call triggers PhoneSessionClient to send its SignalCallCreate signal. You connected to this signal earlier as part of your initial setup. When an incoming call request is received, the PhoneSessionClient creates a new Call object and sends this signal, along with the Call object. Because SignalCallCreate is sent whether you or someone else created the Call object, the only way to find out what caused this call is to connect to the Call object's SignalSessionState signal."
to get more step by step details and sample code, check: http://code.google.com/apis/talk/libjingle/voice_chat.html