Konami had a special site set up for this set. Each card has a digital code on the back. When you registered the code on the site, you could hear a recording of the character. Each regular card had three versions, a, b, c. The images on the cards were identical, but the recording you could hear was different. Rare cards only had one version, but a longer recording.
Konami's site is no longer available. This blog has most of the the text of the recordings: https://ameblo.jp/teninu-memo/entry-11202232736.html
This series of cards was nicknamed “デジプリ” or “Degipuri”