Users on platforms like r/learndutch on Reddit frequently share tips on finding audio files or PDF versions, though availability for the new 2020 edition is more restricted than the older versions.
If you cannot afford a new license, several platforms offer free Dutch audio practice that complements the De Opmaat curriculum: de opmaat audio free new
For listening practice, nothing beats real-world input. Users on platforms like r/learndutch on Reddit frequently