no loudspeaker audio but ringtones ok, after screen replacement

Hi All

I recently replaced the damaged screen of an iPhone4, when it all went back together everything worked fine except for problems with the loudspeaker, which worked perfectly before i started working on it. It is strange though that it will play ring tones perfectly well but no music, video, you tube etc

Tested speaker by changing ring tone - could hear through loudspeaker ok.

I am unable to increase (or decrease) the sound bar using the side control buttons (though could when testing with ringtones)

When making a call, switching to speaker works fine.

Occasionally hear the first two screen clicks when entering lock code.

have checked mute isn't switched on.

Works fine through headphones.

Have checked all settings regarding to the sound and all are set as expected.

Have cleaned all contacts with acetone

Have cleaned speakers with dry brush and air spray.

Have tried changing the speaker unit with no change (though i didn't think it would make any difference as the call on speaker worked)

This is now driving me nuts and i was only doing a pal a favour, and could really do without having to try and explain this to him. So any tips/advise or even guesses would be much appreciated.

Thanks

Andy