@idanroz I changed my MX4 battery 1 week ago and faced the exactly same problem. When I screwed the frame back on I accidentally bent one of the two bronze-coloured metal connectors that are located in the bottom left of the phone. The metal connectors need to connect correctly to the frame. These are what provides the audio to the speaker.
You need to unscrew the frame again and make sure the connectors are properly aligned. That fixed it for me.