Internal speakers have disappeared following RAM/SSD upgrade

I have a 27" mid-2011 iMac - today I upgraded it to 16GB RAM and added an SSD (in addition to the existing hard drive). The memory upgrade I performed myself but the SSD upgrade was done by a qualified tech.
Now my internal speakers no longer appear as an audio output device, instead I have a "digital out / optical digital-out port". I cannot adjust the volume (my menu bar volume icon is greyed out) and no sound plays either through the speakers or connected headphones once I login, however I hear the startup chime when I first turn it on, so this doesn't appear to be a hardware problem - it is perhaps a driver issue? Is there any way I can rectify this without reinstalling OS X? I plan to install Yosemite on the SSD once it's out but would prefer to wait rather than installing Mavericks on it first.
I have already tried resetting my SMC, resetting my PRAM a few times and also reinstalled the 10.9.4 combo update, but neither have resulted in any changes.
Thanks guys.