As a member or a contributor, you can add memories to any selection in a picture with the compelling quality of the human voice. Since ambient noise or computer fans can be a problem, it's best to do this with a microphone or headset. How it's attached will vary depending on the kind of computer you're using but if the quality is good enough for a video call or Facetime, it will probably work fine.
Once you're comfortable just click the record button and you'll see the waveform appear as you record the story.
If you'd like to include a story that was previously recorded or a video story, you can do that with a link. Just find the link (YouTube.com, SoundCloud.com, and other cloud-based media hosting sites work very well,) and paste it into a Ponga picture.
The power of the human voice, or the subtle micro-expressions as a person tells a story can be one of the most powerful elements of storytelling this way. This approach is sometimes called "transmedia."