I am using the free version and I never hear any audio.
Is this normal or is there only audio in the pro version?
Thanks in advance.

If you are not hearing any audio, that just means that the creator of the course you’re taking didn’t add any. Audio is not dependent on whether you’re a paying user or not. You could always try another course that contains audio, but the only way to know if it has any or not is by adding it to your dashboard, checking one of its levels, and pressing “options” then “preview” in the mid-upper right of the level.