I run skype for android 2.3.6 on my Samsung Galaxy S2 (stock firmware, not rooted etc) and have the same issue (+ an annoying workaround).
I experience that I can't change the in-call volume with the phone's volume rocker button. The volume stays the same. The only way of changing the volume is to leave the skype app (so it runs on background) and go to my home screen. There the volume can be changed and when I return to Skype, the volume has changed there too. So with this detour, it can be done, but it's very annoying like that.
Can you please change/fix this, Skype???
The Skype Android App uses the "Media" volume and not the main phone volume. Hopefully, most of you may be aware of this?
Check the Media Volume under . . .
>Settings >Sound >Volume (incoming calls)
Slide to Media Volume to the right towards maximum
This Media Volume will change by the Volume Rocker Button. . . during a call
My Galaxy S2 is clear for me during a call when I have my Media Volume turned up full (full for me as I am a bit hard of hearing). I only find the Skype "Modern Ringtone" (the normal Skype default ringtone) on my Android Galaxy S2 far too soft for my liking when my phone is in my pocket and I am walking in and about normal "street" traffic noise. The Skype ring tone is OK with cable headset, but I do not use them much and the speaker phone is rather soft.
The Media Volume will change the caller's voice volume a bit louder, but the Skype ring tone on the Galaxy S2 speaker phone is very soft and at this point cannot be changed.
I hope these comments are helpful?