First off, welcome to the forum.
Though your experiment may appear to have given results which seem to confirm that talking to plants can be of benefit, you really did not have a big enough sample to come to any definitive conclusion. The fact is that two or more plants can show differing growth patterns even though they have been subjected to identical conditions.
As a case in point, I took 3 cuttings from a Hibiscus last autumn. All three cuttings were the same size and from the same plant. All were planted in identical pots with the same compost and kept in identical conditions. I have just had a look at the cuttings and can report 2 are doing fine with healthy signs of budding, but the third is not doing so well. Though it has survived it shows little sign of budding.
BTW, I have not conversed with any of them.
I don't want to put a dampener on you, but it might well have been that your plants would have shown the same results whatever treatment they were subjected to.
All the same, well done. Perhaps you might like to try again, one day, with a bigger sample and subjecting the plants to both speech and recorded sound. But to come to any meaningful conclusion, your sample will need to be of at least 10 plants per set condition and even then the result could only be considered as suggestive, not confirmed.
As the podcast has suggested, CO2 is probably the greater factor, but vibration may also play a part, artificially tricking the plant into strengthening to be more resilient against wind.