MBA early 2015 no sound, preferences speaker symbol grey, no device

Hello

I have no sound and no speaker symbol, as well in the system preferences --> no device

absoluty no sound also at the boot up sequence,

tried a lot, hints from here as well and all I found on the www, so far

jack with headphones, pram, smc, different iOS-versions back-and-force left-and-right up-and-down, live boot linux, windows etc.

but I have sound via thunderbolt/display-port --> hdmi

what I learned, the audio-controller-chip is on the i/o-board and if the chip is damaged

= no sound !!!!

haven´t ordered i/o cable and i/o board up to now would be the next step´s

but have a question to the pro´s out here,

is there a different rail via thunderbolt to deliver sound ?

thank you in advance