There is lot of internal sound available in the market. If you able to bought some, try to play and compare the different between internal sounds in your collection.
Technically, most of sound will be mixed between mating sound, baby sound, parent sound and etc.....the best if you have a few sounds that you can try to play and investigate which will make happier and calmness to bird
I've a quite number of internal sounds. Previous internal sound used at my birdhouse show stagnant growth which number stop at 15 nests.
I make the decision to change the internal sound....with hard-hearted ...and no u-turn ....now I start to see a progress after the new sound been played. Bird more attractive to this new internal sound