skype video calls speaker volume low

Check your media volume level. Should be found in your phone's settings, under sound and vibration...... or something like that.

 

https://f-droid.org/en/packages/com.roozen.SoundManagerv2/

 

It sounds to me, if it is the vume of the loudspeaker itself changing and it is not actually switching to the earpiece speaker, that it may be caused by a bandwidth issue.

Video is sure to use much more bandwidth than audio alone.
So the audio will decrease in order to make room for the video content.

This may be a setting in the app, it could be network constraints, or it may be device limited somehow.

If it is a new problem, then the device can most likely be ruled out.

If the app or device was updated right before the problem existed, or your network was changed somehow right before the issue began, then Bingo.

Also, depending on the time of day may matter as well.
The volume of videos that I stream changes throughout the day and night as the total amount of useage for my provider changes.

And this is true with the fastest provider around here, let alone any of the others.