Speakers are tinny unless pressure applied to bottom of the case

I normally use my external speakers, so I hadn't noticed this until recently. But I suspect this happened after disassembling the Macbook to replace a broken trackpad.

The speakers sound extremely tinny and seem to only be coming from one side. When I put some pressure on the bottom case cover (pressing the bottom with my fingers / hands), the speakers sound like normal again.

Should I disassemble the laptop again and make sure everything is correctly reassembled?. Where is the speaker connection?