iMac - No audio on internal speakers

[randomjerk2]

I'm facing a really strange issue that has existed on various versions of Mint(from 17.x to 18.2) and even on Ubuntu 18.04 currently. I have an iMac 21.5 running 18.04 with Kernel 4.16.12 with the audio settings below.
https://i.stack.imgur.com/NX9OQ.png.

The issue is that in-spite of "Speakers - Built in Audio" being selected as the default device, I don't hear any sound coming out of the iMac speakers most of the time. It works sometimes. Chrome seems to be having this issue more often than Firefox or any other media software. When it doesn't work, looks like the sound goes through to HDMI - Display Port 3.

Can someone please help me in how to make sure the default option of "Speakers - Built in Speakers" can be made to 'stick' ?

Here are the outputs of aplay -l and pacmd list-cards. Inxi details of the machine below

https://i.stack.imgur.com/HUmc7.png

[wildmanne39]

Thread moved to Apple Hardware Users for a more appropriate fit.

[wildmanne39]

Please use the attachment facility by clicking on the paperclip or use url's for images because apart from bandwidth issues for some, it makes it difficult for those using mobile devices to browse the forum.

[randomjerk2]

Please use the attachment facility by clicking on the paperclip or use url's for images because apart from bandwidth issues for some, it makes it difficult for those using mobile devices to browse the forum.

Thanks. Will do going forward