Someone records a conversation on their phone in which they receive a threat; an employee records their boss coercing them; an extortion victim records the extortionist’s call. Can those recordings be used as criminal evidence? Audio recordings generate a great deal of confusion: they seem like ideal evidence, because they capture what was said in the speaker’s own voice, but obtaining them can brush against fundamental rights such as the secrecy of communications and privacy.

Article 18.3 of the Constitution guarantees the secrecy of communications except by judicial decision. This right protects communications while they are being transmitted, both their content and the very fact that they exist. It is worth knowing that it does not protect in-person conversations with the same intensity as telephone ones, a distinction that has practical consequences.

The key lies in a distinction established by the Supreme Court: recording a conversation in which one is oneself a participant versus recording a conversation between others. When a person records a conversation in which they are an interlocutor, they do not violate the other party’s secrecy of communications, because that right protects against interception by uninvolved third parties, not against recording by one of the participants. It is lawful even if the other person does not know or consent to it. That said, that lawfulness does not guarantee admissibility: it may collide with the right to privacy if the conversation touches on especially intimate matters, and the court must carry out a proportionality assessment. By contrast, recording a conversation between others to which one is not a party does violate Article 18.3: such recordings are unlawful and null unless judicially authorised, and may constitute an offence of discovery and disclosure of secrets.

For one’s own recording to be admissible, additional conditions must be met. Authenticity: whoever submits it must be able to establish that it has not been manipulated, ideally through a notarial record or an audio forensic report. Proportionality: the court assesses whether the recording amounts to a disproportionate intrusion into privacy, although most recordings of threats or extortion pass that test. And the identification of the interlocutors, which may require voice expertise if the defendant denies being the one speaking.

Recordings made by the police are subject to stricter requirements: the interception of communications always requires prior, reasoned judicial authorisation, without which they are null.

It is worth remembering what a recording proves and what it does not. Context is fundamental: expressions that seem like threats may be, in another context, expressions of anger with no real intent. And a recording only proves what was said, not the truth of what was said.

Faced with a recording that harms the defendant, the defence may challenge the legality of how it was obtained, for example if it was made by someone outside the conversation; question the authenticity and integrity of the file by requesting a forensic audio examination; offer an alternative interpretation of the content; and point out what the recording does not contain that would be expected if the events had occurred as the prosecution maintains. If a recording has been edited to remove parts favourable to the defendant, that manipulation may entail criminal liability and the exclusion of the evidence.