Under what circumstances, if any, can audio assistive technology be used in confession? This recent answer mentions that it is not permitted to use the internet or cellular communications in (private) confession. It also mentions a story of a certain catacomb with particular acoustics that allowed the priest and parishioner to be widely separated.
Now, the answer (and in a sense, its related question) is dealing with desirable separation (e.g. for health reasons) between the priest and parishioner, which obviously raises a potential issue of them being able to hear each other.
I understand why that digital networks are not allowed because they allow for the possibility that the audio might be intercepted, thus breaking the seal of secrecy that is supposed to exist. (Side question: are confessionals ever swept for bugs? ) However, this leaves me wondering, would a closed circuit system be acceptable? I'm thinking something like a microphone on each side connected to a speaker on the other. (Probably through an amp, but the amp would not be connected to anything else other than power. Thus, the chance of the audio being &quot;intercepted&quot; somehow not be appreciably increased. The simplest version of this, I suppose, would be some sort of closed-circuit telephone system such as what's seen on TV when someone is talking to an inmate.)
p.s. Peter Turner raises an interesting point, which is that one would think the answer to the above would be the same as whether or not a person using electronic hearing aids can partake in confession. I would rather expect there exists an official ruling on that, but I also wonder if that has been extended to other systems such as I discuss above and/or if there is a broader ruling on this general subject outlining what is and is not permissible.
Under what circumstances, if any, can audio assistive technology be used in confession?
Before answering this question further, I would like to mention an important point. Visual contact and/or a physical presence, even if behind a screen is absolutely necessary.
Generally speaking there are no sacraments that can be received over modern forms of communications, including over cell phones, etcetera. You can not baptized someone over the phone.
Nevertheless some confessionals, I have been in, are equipped with a form of hearing aid strictly for use between the confessor and the penitent only. As such secrecy is maintained. Again, a physical presence equally been maintained.
Two way phone calls, in high security prisons have been validly used by priests in prisons to here confessions. This is equally a question of necessity, as state rules will not permit physical contact of inmates with anyone visiting them. Again the inmate is visually present, even though thick glass and/or bars that separate the two.
In the article, The Monk and the Murderer Fr. Kirk Larkin heard the confession of James Malicoat on the day he was executed at Oklahoma’s States Penitentiary infamous H-Unit on August 31st, 2005.

But at 10:45 a.m., under their own tension, the monk and the priest were ushered in to see Malicoat. After days of trying to conjure up what the killer would look like, Father Larkin was relieved to see a human being on the other side of the glass. Brother Vianney-Marie had never seen him look worse. Malicoat was slumped over for terror and want of sleep. &quot;It's inhuman,&quot; the monk said. He was given about an hour with his last visitors, and Brother Vianney-Marie wasted no time. He made quick inquiries about Malicoat and his family and then introduced him to Father Larkin, telling Malicoat that he could make an act of faith and confess to him. He gave Father Larkin the telephone.
Malicoat told the priest he didn't want to confess.
&quot;James was truly concerned about other people more than himself at this point,&quot; Father Larkin recalled. &quot;He told me, 'Father, I don't want to burden you with the horrible things that I've done and that have been done to me over the course of my life.'&quot;
The priest, with nothing rehearsed, trying to pry open the window that Brother Vianney-Marie had cracked, turned to the Profession of Faith. He went through the Creed point by point, asking Malicoat whether he agreed with each article.
Brother Vianney-Marie, his active participation relinquished, could hear only the priest's side of the conversation. He was in agony. When Father Larkin turned to apostolic succession and how a priest could forgive sin, the monk imagined how much time the explanation could eat up and couldn't take it any longer. He got up and went to the back of the room, where he paced and prayed the rosary out of earshot.
Brother Vianney-Marie looked up from his prayers in H-unit to see Father Larkin raising his hand in blessing over James Malicoat. The monk had no way of knowing whether this was absolution. But he could be pretty sure.
&quot;I just knew it was his confession,&quot; Brother Vianney-Marie said. &quot;I was ready to jump through the ceiling.&quot;
A few minutes later and Father Larkin was beckoning Brother Vianney-Marie back to Malicoat. Their half an hour apart had closed a chasm. And opened the window.
&quot;All of a sudden, he had this weight off his conscience,&quot; Brother Vianney-Marie said. &quot;I told him, 'I don't think you're my friend, you're my brother.' He has all the same graces. I said, 'Are you ready to go?' and he said, 'Yeah.' There was a peaceful tranquillity. He realized what he had done and was man enough to accept the consequences.&quot;

In the 23rd photo of the Tour of the Oklahoma State Penitentiary in McAlester - Photo Gallery one can see the location where Fr. Kirk Larson brought James Malicoat into the Church and heard his confession.
Fr. Larkin had maintained visual contact with James the entire time.
Further information may be gleaned from the following article:

Confession by phone, Skype, or emoji? Could it happen during coronavirus pandemic?
Online confession and absolution
Can I Make My Confession Over the Phone