Well in my opinion I experienced this on my iphone 6 about a week ago before I updated and got error 53. If you press on the screen does it pop up and down like it's loose and not tight? Well in my experience I believe the screen is bad and isn't supporting enough pressure down to where the loudspeakers pins are pressured onto the charging port connector. I know this because I tested my other old lcd screens and they were tight and weren't loose not to mention the speakers were working fine and siri was able to hear me. My newest recent screen caused siri and my loud speakers not to work and my glowing apple logo. I tried everything from using another brand new loud speaker and new charging port and still the same. After testing out the glowing kit for the iphone 6. I knew my newest lcd screen was the problem to siri, my loud speakers and my glowing logo. A pic below to show the glowing logo illuminating. Just make sure you get a good screen. Some screen really do have problems closing correctly.
This is a no backlight issue. I had this issue recently it is caused by having the iphone lcd on while changing lcd. I shipped mine to the United Kingdom. They did a great job fixing my iphone backlight. I used jailbreak so everytime I slide my phone it made the old unlock sound. I noticed mine was on and working because I heard it unlock just it was a black screen with any screen I put onto my iphone 6. Here is a link if you need it fixed. Took about less than two weeks to get mine back. http://www.mobiledeviceworkshop.co.uk/ip...-6-backlight-coil-fuse-filter-ic-repair-service-3399-p.asp