Yes, there really is. Even here on Duolingo, you can hear the difference between leabhair and the leabhar in this exercise.
When a word is said quickly, or is followed immediately by another word, it may be difficult to hear that difference, and there can be differences between dialects and individual speakers, but the difference is there, even though English speakers often don't notice it, unless they are specifically paying attention to that particular sound.
The current Duolingo speaker is usually reasonably clear with her slender r (though it's easy to ignore unless you are actively listening for it), and there clearly isn't a slender r in this exercise.
Unusually, teanglann.ie actually has a pronunciation of the plural leabhair, but only the Munster speaker users the leabhair version of the plural, with a distinct slender r - the Connacht speaker uses the Connemara leabhra variation, and the Ulster speaker uses a different plural again.
But there is also the phrase léirmheas leabhair ("a book review") with the genitive which is also leabhair - in this case you can hear the slender r in all 3 dialects.
In a nutshell: to say that you have something in Irish, you say that it is "at you", while to say that you want something, you say that it is "from you".
- An bhfuil leabhar acu? Do they have a book? (literally, Is a book at them?)
- An bhfuil leabhar uaibh? Do they want a book? (literally, Is a book away from them?)
"An bhfuil" is the question part, which frequently translates to "do", even though the hints suggest "is? / are?".
Look at it like changing a statement into a question "Tá leabhar acu." is changed into "An bhfuil leabhar acu?"
To confuse: The negation form requires the "l" in "níl": "Níl leabhar acu."