RaspberryPI and soundmodem PTT

The RPi has one built-in serial port but in it's default mode, it only supports TX / RX and GND pins.  That won't help you for PTT. Now, what you can do is invoke one of the RPi's alternative I/O mappings and at that point, you have full access to the flow control pins (specifically RTS) that most people use for PTT.  Please note, the RPi is a 3.3v device with limited current so you'll probably need a level shifter to drive the PTT line on your radio.

http://lavalink.com/2012/04/more-on-raspberry-pi-serial-ports/

Ps.   In addition to considering Soundmodem, have you heard of Direwolf?  This soundcard TNC supports is being actively developed, supports multiple parallel decoders for off frequency signals, supports multi-bit correction, etc.  None of those times is true of Soundmodem.