Manual of Indulgences (USCCB 205) shows norm 24:
N24. Confessors can commute either the prescribed work or the conditions in favor of those for whom these are impossible because of a legitimate impediment.
Handbook of Indulgences (USCCB 2005)
30.1 A plenary indulgence is granted to the faithful who read the Sacred Scriptures as spiritual reading from a text approved by competent authority and with the reverence due to the divine work, for at least a half an hour; if the time is less, the indulgence will be partial.
30.2 If for some good reason a person is unable to read the Sacred Scriptures, a plenary or partial indulgence is granted, as above, if the text of Sacred Scripture is listened to while another person is reading or if it is heard by means of a video or audio recording.
Then there is prayer. It is possible to receive a partial indulgence by mental prayer, utilizing Concession 15, Mental Prayer.
Other prayer, it is to be vocal prayer (it should a least be audible to oneself, such as a whisper).
In the Manual of Indulgences (USCCB 2005), the requirements for the plenary indulgence of the Marian rosary use the words “recite” and “vocal prayer”. Recite is audible according to the dictionary.
In the general norms we see that the deaf and mute are not bound to this in public or private (N26 below) [emphasis mine]. They are very specific here to detail the requirement of recitation, with an grant for the deaf and mute to “recite the prayers mentally”.
N22. An indulgence attached to a prayer can be acquired by reciting the prayer in any language, provided that the translation is approved by the competent ecclesiastical authority.
N23. To gain an indulgence it is sufficient to recite the prayer alternately with a companion or to follow it mentally while it is being recited by another.
N26. Both the deaf and the mute can gain indulgences attached to public prayers if, together with the other faithful praying in the same place, they devoutly raise their mind and affections to God; regarding private prayers it is sufficient for them to recite the prayers mentally or express them with signs, or simply to read them without pronouncing the words.
You might have noticed from the Catechism that the emphasis is on the perfect form of prayer.
Catechism of the Catholic Church 2703 states that
“He also wants the external expression that associates the body with interior prayer, for it renders him that perfect homage which is his due.”
In a plenary indulgence we strive for a perfect expression, falling short we have a partial indulgence.
Mental prayer is not the same as reciting because of external expression. Vocal prayer (recited) also requires mental engagement. As the norms mention for indulgence there exists at the time some vocal expression because
“it is sufficient to recite the prayer alternately with a companion or to follow it mentally while it is being recited by another.”
Catechism
703 “He also wants the external expression that associates the body with interior prayer, for it renders him that perfect homage which is his due.”