1- Check the positioning screws of speaker 2- Check Speaker and cable contact points. 3- Change speaker. 4- Change front camera flex cable. 5- Check connector of front camera and speaker on logicboard.
After replacing the LCD screen, you use a longer screw to install conectors protecting the sheath . It causes damage to the logicboard . Your troubles are similar to the "blue" screen trouble on the ip5s. I am a technician specializing in repairing broken glass display LCD's iPhone and have encountered this case many times, and have found the cause, overcome the above error.