Some recording applications create m4a files. Boom can play those files, but have seen some bugs in various browsers (Chrome) where these sounds don't play. The answer is to convert these files to mp3 files. There is a website that will do these conversions for you, for free called Cloud Converter. Here is how to use it.

Go to Cloud Converter  in your browser. It should look like this:

If you are converting more than a couple of files, you will need to Sign Up. You can use your Google login for this, if you want to save some time. Once you have completed that, you should be back at the main screen. Now you can select and upload the files you want converted. To do that, click on Select Files > Select from my computer

This opens a standard file dialogue. Navigate to your the folder with your m4a files and select one or more files (use shift-click to select multiple files), then click "Open" in the bottom right of the dialogue.

You should see your files on the website:

The next step is to tell Cloud Convert what format you want the files to be converted to. Click on: convert all to m4a > audio > mp3 (see the screenshot below):

I've circled the menus in red where I've already clicked on the correct selection. The screen should look like this when you are done:

Now you are ready to convert your files. If you have a Google Drive or Dropbox that you would like to save your files to, click the little box at the bottom of the screen. Otherwise, your files can be downloaded to your computer. Click Start Conversion and each file will be converted. You will now see a Download button next to each that has been converted

Click Download and the file will be saved to your default Download location. Now you can upload those mp3 files to Boom and use them in your decks.