Yes, you can listen to subliminals with one earbud or earphone. While not as effective as using both of your ears you will still absorb the affirmations recorded on the subliminal.
If you have a broken pair of earbuds or headphones with only one ear working you can still use those.
In some cases you will want to listen with one ear while you are at work or doing other activities where you need to be fully aware of your environment.
Like I said the best way is to listen with earbuds or headphones in both ears at the same time to get the full effect of the subliminal. Listening with only one ear you will not get the effects of the binaural beats.
The binaural beats recorded on our subliminals place you in a very relaxed state of mind where the affirmations are easily accepted by your subconscious mind.