It is not thought that is judged, only what is expressed and done. In other words, your actions are what counts and that includes expression of thoughts whether through speech, writing, or other media. For example you cannot just think about the shahada, you have to express the words audibly to witnesses. The same applies for performing dhikr. As with salah, it is more sebab to perform dhikr in a group (two or more) where you can hear and witness each other's dhikr. The external manifestation of your thought and intention is the confirmation of it to yourself and to other witnesses. If you are performing salah by yourself you are only getting 1/27 of the benefit than what you get by performing in a group according to sahih hadith. When you are performing salah by youself, then you must recite the words of the salah audibly to yourself, doesn't have to be loud, just enough for you to hear them.
To better understand this it is helpful to get into some ontology and look at the 10 spiritual portals of the heart: the mouth, the nose, the ears, the eyes, the face, the head, the neck, the hands, the feet, and the gender organs. These portals to the heart must be purified through wudu (Quran 5:6) and/or ghusl before one can perform the salah, or touch the muzhaf (Quran 56:79).
In order to complete a valid act of worship there needs to be an output from a suitable portal from oneself or others, matched with an input from a portal. If you are performing dhikr by yourself, you ouput the dhikr through your mouth by speech, so that you can audibly input through your ears, completing the loop, confirming and strengthening the spiritual heart and getting closer to Allah.
If the worshipper is deaf and/or speech impaired and is alone by his/her self, then with Allah's help he/she needs to find a way to use the other suitable portals to perform the dhikr. Its easier and more sebab to just participate in group dhikr so that the speech output is done by others, and you just input. If you must do the dhikr alone, where there is a will, there is a way.
The late physicist Stephen Hawking lost 90 % of his neural muscular motor control, competely paralyzed from Lou Gehrigs disease, but that did not prevent him from his great accomplishments in physics. Worshiping Allah swt is even more important. That is why Allah swt created mankind and the jin, to worship Allah swt.
