According to the opinion that holds kesivah kedibur, there is another manner in which words can be considered as spoken and not as mere thoughts – namely by writing them. Written words are considered as if they are spoken, even though no one actually heard them. Therefore, even regarding a situation where optimally one is required to hear what he is saying, one could apply the halacha of kesivah kedibur since the written words will now be considered spoken words and not thoughts.
I believe that Reb Akiva Eiger’s issue would apply even if he held the opinion that the halacha that one must hear what he is saying is merely a means to render the words as if they were spoken. This is because even the opinion that one does not actually need to hear one’s self nonetheless supports the belief that the words have to be spoken on some decibel that could theoretically be heard in a quiet place. Written words, however, can never be heard; thus we could not apply the concept of kesivah kedibur to a situation that requires one to hear himself.