Dictation for Index Card 4, and most other iOS apps, is handled by the iOS Siri feature. This is available on iPad (3rd generation) and iPhone 4S and newer devices. Tap the microphone icon on your keyboard to enable dictation of card text.
If you do not see the microphone icon, make sure that Dictation is enabled in your device settings:
Turn on Dictation: Go to, iOS Settings App > General > Keyboard > Dictation.
For additional info on enabling this feature, check out Apple Support. This link may help you:
There are also some tips on sites like iGeeksBlog that might help with common issues:
Note that Apple does not allow third-party keyboards to access the microphone: