The play[] function casts its first argument to a string, so you can use "," to join together several string fragments and add a prefix or suffix:
play["dima_",me.text] play[me.text,"-of-dima"]
You could alternatively use "format", which is a little more complicated but much more flexible:
play["dima_%s" format me.text] play["%s-of-dima" format me.text]
To support multiple voices, you then need some way of identifying the current speaker. Currently you're using different cards for different speakers, which has its own advantages, like allowing for character art or special catchphrase buttons or something. If the card names corresponded to the prefix for sound clips you could do something like
play[card.name,me.text] # in the phoneme buttons play[card.name,word] # in the "play" button
Or you could simply use different versions of the scripts on different cards; whatever you find simpler.
One note that may be helpful if you aren't already aware is that if you select multiple widgets you can edit all their scripts at once by choosing "Widgets -> Script..." from the menu.
