Audio Playback: Missing vowels
"eh" produces a broad range of frequencies from below 100 Hz to over 7 kHz according to my Rode Microphone and spectrum analyser. That's little "k", big "H" and little "z" for kHz in case you didn't understand what I was suggesting!!

Why don't you use your PC to record your talking and analyse it in one of the freebie wav editors you can get to double check. You could also try outputting some pre-recorded speech as an audio input that your pic can try and reproduce to see if that helps.