Low Volume in Exported MP3 file on version 2..13

I am very new to using Audacity 2.1.3 and so far I have managed to convert some very old story cassettes that my children use to listen to, to MP3 files for my grandchildren to listen to. I can hear the ‘stories’ clearly when I replay them on Windows Media Player, but when I send them to my daughter to play she can hardly hear them. I send the files to her drop box to prevent distortion and compression but despite several attempts we can not identify what is going wrong. My daughter is ‘playing’ the files with the ‘volume control’ at the highest levels.
As we are both very new to this area I am hoping someone will explain in ‘not too technical language’ how we can overcome this problem and what I/she is doing wrong.
With thanks

The correct levels for voice look similar to this …

the dark-blue part peaks at about 0.75, and the light-blue part peaks at about 0.25 …

If the audio files you send look similar in amplitude, then the problem is at her end.

https ://manual.audacityteam.org/man/audacity_waveform.html

The Amplify effect will pre-scan your file than then automatically adjust to whatever gain is needed to normalize (maximize) the volume. The Normalize effect works similarly.