As discussed in the previous posts, the DFPlayer Mini has a maximum volume setting of 30, and the speaker is connected through a 3 watts amplifier.
Moreover, different speakers have different volumes, so I have chosen the best one, which was the Iphone XR or Iphone 12 Pro Max loud speakers.
However, the sound volume can be improved also by using software.
So I have performed 2 processes:
1) Increase the volume to the maximum possible in the TTS service. So that the MP3/WAV file that I download is already the best.
2) Use Audacity's function "Amplify" to increase the volume of the audio file by an amount of decibels. However, too high of a number and there is clipping with the audio quality suffering.
In the following image, 5 tracks can be seen, representing the same audio voice command.
The first one is the original.
The second one has been amplified by 2db.
The third one has been amplified by 4db.
The forth one has been amplified by 6db.
The fifth one has been amplified by 8db.
Here is the output from the device:
By checking the sound quality itself, it seems that 4db is good, and 6db is too much.
And considering that the original file is loud, but could be louder, a 5db amplification would prove optimal.