Mackenzie Shirilla says she's bored to tears behind bars ... but she might want to get used to it, considering she's serving 2 consecutive life sentences.

TMZ obtained audio from a prison phone call between Mackenzie and her mom, Natalie, where the convicted killer complains about how slowly time is passing in lockup.

You can hear Mackenzie's annoyance ... she doesn't have access to her commissary and complains about having one book she doesn't want to keep reading over and over.

At one point, Mackenzie groans about the time of day.

When her mom asks if she could get a job mopping or sweeping around prison to kill some time, Mackenzie explains they won't let her have one ... at least that's what she's heard.

She said ... "They don't let people with charges like me get jobs," before launching into gibberish that appears to explain why she hasn't asked the staff about it.

Remember, Mackenzie is serving time for murdering Dominic Russo and Davion Flanagan ... and isn't eligible to go before the parole board in Ohio until 2037. She's fighting her conviction and says she's innocent.

Don't worry, Mackenzie ... Gypsy Rose Blanchard told us one way to pass the time behind bars!!!