Can I use android phones and PC as walkie talkie? (without internet!)
Install SIP user agents on both phones and PC.

You can use SipDroid or CSipSimple for Android and a softphone program like X-Lite on the PC.

Note that you need to set up local accounts when configuring the software. You can then call other parties on the local network directly by their IP:Port.

If you don't want that, you need to install a full blown VoIP proxy like asterisk.