Audio recordings as evidence: when they are valid in a criminal trial
Someone records on their phone a conversation in which they receive a threat; a worker 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 lot of confusion: they seem like ideal evidence, because they capture what was said in the speaker's own voice, but obtaining them may touch on 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 with practical consequences.
The key lies in a distinction established by the Supreme Court: recording a conversation in which one participates versus recording a conversation between others. When a person records a conversation in which they are a participant, they do not infringe the other party's secrecy of communications, because that right protects against interception by unrelated third parties, not against recording by one of the participants themselves. It is lawful even if the other person does not know or consent to it. However, that lawfulness does not guarantee admissibility: it may clash with the right to privacy if the conversation affects especially intimate aspects, and the court must make a proportionality assessment. By contrast, recording a conversation between others in which one is not a party does infringe Article 18.3: those 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 provides it must be able to establish that it has not been manipulated, ideally through a notarial record or an audio expert report. Proportionality: the court assesses whether the recording entails a disproportionate intrusion into privacy, although most recordings of threats or extortion pass that assessment. And the identification of the speakers, which may require voice expertise if the accused denies being the one speaking.
Recordings made by the police are subject to stricter requirements: the interception of communications always requires prior and reasoned judicial authorisation, without which they are null.
It is worth remembering what a recording does and does not prove. Context is fundamental: expressions that seem like threats may be, in another context, expressions of anger without real intent. And a recording only proves what was said, not the truth of what was said.
Faced with a recording that harms the accused, the defence may challenge the legality of how it was obtained, for example if it was made by someone unrelated to the conversation; question the authenticity and integrity of the file by requesting a forensic audio expertise; offer an alternative interpretation of the content; and point out what the recording does not contain and 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 accused, that manipulation may carry criminal liability and the exclusion of the evidence.
