The aim of the study was to assess the behavioural reaction and emotional arousal of warmblood horses in response to sounds of different origin, and to classify those sounds into neutral ones, those causing a behavioural change and those causing a behavioural and physiological stress response. We tested the hypothesis that the perception of a sound as neutral or potentially threatening does not simply depend on the sound origin per se, but rather on the context in which the sound occurrs (predictability), additionally enhanced by the novelty effect. Recordings of 40 sounds, known (KS) and unknown (US), from four groups: anthropogenic (AS), neutral animal (NAS), predator (PS), and inanimate environment sounds (IES), were played to 20 warmblood horses remaining in their familiar paddock. The duration, frequency or occurrence of certain behaviours (e.g. walking, feeding, standing alert, stopping current activity), the heart rate (HR) and heart rate variability (HRV) were measured. The horses’ reactions were rather weak and short-term. Most of the sounds resulted in distraction (increase in alertness). The horses ate less, walked and stood alert longer, and had a higher HR after certain sounds were played. The strongest stress response (physiological and behavioural) to NAS, mostly US, was observed. PS caused behavioural disturbance, but no cardiac activity changes. The weakest responses were observed for AS and IES. Modern, stable-kept horses remain vigilant to auditory stimuli in their environment and differentiate their responses to different sounds. The perception of a threat posed by sounds depends on their unpredictability and novelty.