In the same way as for Windows, you need to set up your sound card to make the most of the possibilities offered by your installation, i.e. the speakers built into your iMac/MacBook or an externally connected 5.1 system for example.
Start by opening Audio and MIDI configuration via Spotlight.
Default use case: built-in stereo speakers
In the left column, click on Integrated output. In the right column, use both drop-down lists at format level to define the maximum sound quality i.e. 96,000 hz, 2 channels, 24 bits.
You can also set the power in dB (decibels) of your speakers separately.
Use case: external sound card + set 5.1
It is quite regrettable that the iMac does not integrate (even optionally) a sound card 5.1 see 7.1 as is the case on any basic PC. One solution is to plug in a 5.1 sound card externally compatible MacOS: and the builders don't rush to that level. Hercules offers the Muse XL Pocket LT3, only sound card to date (except my mistake) able to make 5.1 on Mac, all in jack cables.
Even if their site indicates that the map only offers stereo on Mac, I will show you the opposite of that step. In the window Audio and MIDI configuration, click on Muse Pocket LT3. In the right column, click on the tab Output if the latter is not active. As before, use drop-down lists to define sound quality, either 48,000 hz, 6 16-bit channels for 5.1. Here again, you have the option of adjusting the sound of each speaker separately (rather convenient to boost the rear for example).
Note that the sound card is not capable of producing a quality above 16 bits (e.g. 24 bits), which is the case on Windows. It’s too bad...
Then click on the button Configuring Speakers.
Check first (if not), Multichannels (top) and select from the drop down list 5.1 surround.
This window allows you to test the positioning of each speaker. To do this, click on centre for example, to hear a sound in this speaker only (think to lower the sound before clicking). Then use the drop-down lists to change the order.
Click on Apply then Finish to validate your changes.
Finally, don't forget to set your sound card as the default output audio device:
Now insert a DVD into your Mac’s optical drive and play it using VLC, downloadable for free below:
Why don't you use the DVD Player app? Strangely, only VLC is capable of restoring sound 5.1.
Your installation, stereo or 5.1, is now correctly configured: you get the best sound quality ever!