Most transcription software will automatically loop; or at least easily let you loop a section of audio such as someone making the "ooh" sound in "Today". It will do this easier than a full-blown waveform editing program.
Doing this inside transcription software will save you from having to manually edit the track, loop it, etc. These programs will allow you to simply highlight the section of waveform you like, and through most defaults automatically start playing back the audio looped over and over. And over.
This can be done as suggested in Audacity and other audio programs, however I find it's just simpler to highlight the selected part of a waveform, hit the space bar and hear that section of audio over and over again.
My experience with this is mostly in Transcribe!, or Capo, but they all act similarly.
Now, when you want to export this away from the transcription software:
However, the audio will probably "clip" in the sense that the noise starts over, and will have an abrupt beginning. In that case, copying and pasting the audio clip inside an editor and Cross-Fading should leave a stream of vowels as indefinite as you like. Or any pitch for that matter.