Find an ambient sound file that is pleasant to listen to; something that goes well with your binaural track or tracks. The Freesound Project is one place to acquire such sounds, and they are free to use. Download several ambient tracks you like. Longer files are better.
Download an audio-editing program, if you do not have one. Audacity is a free and powerful sound editor that works on most operating systems. Install the program; open it.
Open the binaural audio track in the editing program. Play it to ensure the right track is open. Import one of your new ambient files in this same project window. In Audacity, this is done by clicking "project" at the top and then "import audio."
Play the new combined file. You should hear the binaural tone as well as the ambient sounds at the same time. Adjust the volume on each track as needed. If one track is far shorter than the other, you can loop it by selecting (highlighting) that track, then clicking "copy." Move the cursor to the next blank area; "paste" it into place. Repeat as needed.
Export the entire new selection as a new file. Give it a new name that won't be confused with any other title. Save it to a memorable location. Open and play the file again to ensure it functions as desired. Play it again in headphones to ensure both left and right channels are functioning properly.