Inventors wanted

[Pira1298]

What about 4 fixed wheels mounted directly onto the back of the speakers, one set of two down low, close but not quite on the ground. To move the speakers simply tip the speaker back so the wheels contact the ground. The second set of two wheels could be mounted close to the trailer floor height. Roll the speaker up to the trailer (on the lower wheels) and then tip it into the trailer (onto the second set of wheels) and roll it to the required position inside the trailer. You could leave it lying down to save your back or pick it up to save space. You could play around with the exact positions so that the speakers are balanced and the wheels take most of the weight.

Bunnings have the wheels you would need.

The trolley just seems like an extra complication.