How to correctly diagnose a motherboard malfunction?

Question

Iron

Good afternoon.
Mat. board: Gigabyte GA-X79-UD3 (Rev. 1.0)
Power supply: Corsair CX750M
Expansion of the disk subsystem. While connecting the power to the new HDD, I took a cable for another PSU (with a different pinout). As a result of the start, I got two dead HDDs (but we are not talking about them) and suspicions of a faulty mat. fee. The symptoms are as follows:
Matt. the board successfully starts the system. SATA are alive. PCI-E are alive. USB did not check everything.
The audio outputs (front/rear) do not work. It manifests itself as a complete absence of sound in the speakers when playing sound from the system (I also checked the speakers separately). At the same time, if the computer is turned off, then randomly increasing / decreasing / breaking noise appears in the speakers. If you remove the speaker plug or turn off the PSU - the noise disappears. There is no noise during operation of the system.
In principle, I have already come to terms with the loss of an integrated sound card, but I would like to hear the opinion of experts on the following issues:

• How to check that the other components of the mat are working. fees?
  
• What can be the result of its further use?