With more people than ever before living in cities, urban parks have become an important way for people to connect with nature. Whilst the visual features of natural settings may be an obvious focus, the sounds we hear are also an important means to experience nature. Studies show that people generally find natural sounds, such as bird songs, pleasant and relaxing. Bird sounds can improve our mental well-being by reducing stress and offering psychological restoration. However, whilst bird sounds are commonly heard in urban parks, people also typically experience some degree of man-made noise, such as vehicle traffic. In this study, I investigated how people respond to both bird sounds and traffic noise in urban parks and how the presence traffic sounds might affect the mental health benefits from hearing bird sounds in urban parks. I did this by having 162 participants listen to 8 different park sound recordings and then answer questions about the bird and traffic sounds they heard in each recording and whether they considered the sounds to be relaxing or restorative. The results of this study showed that park sound recordings with more bird sounds and limited traffic noise offered the greatest potential for mental health benefits. Interestingly, there was also greater benefit the longer participants had lived in the city and as they perceived more types of birds. Traffic noise was consistently rated as unpleasant and, importantly, was found to reduce the potential benefits of bird sounds. We hope these results can help inform urban green space design so that people can experience more bird sounds and less traffic noise when visiting parks. This, in turn, can also provide conservation benefits by promoting a richer array of birds in parks.