Playing flac files on my Mac book

I want to use my Mac book as a music server to take advantage of the spdif to optical input on my dac. 

I transfered all my flac files to the mac into a folder. I have tried VLC and Windows Media but the sound quality is very poor and highly distorted sounding.

I have gone to quite a bit of effort and expense to update this Mac but am very dissapointed. 

Where have I gone wrong. 

Thanks for any help you can offer. 
I should add. This Macbook does have the spdif/optical shown in the list of outputs. Thanks
Probably the best source of help for problems using VLC on a Mac to play FLAC files would be in the support area of VLC's developer.
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my SPDIF/optical no longer functions on my MacBook Pro. I believe the capability was taken away last year (?) with an OS update.
Normally I use Roon, but on my working computer I used to use Amarra, but recently tried Audirvana, and must say I like it better than the newest version of Amarra, Audirvana sounds fine and looks very stylish too.

So the problem is the distortion even at low levels. I tune files  play fine.
Has anyone else experienced this? 

I will check the mp3 files. But I suspect they will play fine.