Thanks again, Bastian. No, yours isn't an stupid question; checking the microphone was the first thing I did, and also did what is explained in this summary:
In my cell Oukitel K6000 PRO with Android 6 everything went fine, but suddenly a problem came up: all audio files recorded in my cell using the Sound Recorded app or WhatsApp voice recorder (I assume WhatsApp has its own) cannot be heard fine when played in my cell because of a very very low volume. However,
1) When using phone calls, I can hear and I can be heard
2) With Google DUO audio, I can hear and I can be heard
3) With WhatsApp calls, I can hear and I can be heard
4) Music I have in my cell can be heard fine too.
If using File Manager I transfer to Music folder any of the several WhatsApp audio files that have volume problems when I play them in WhatsApp, they sound fine with the Music app.
Do you think that uninstalling and reinstallig WhatsApp would help?