It is not possible to convert subtitles (.srt) to audio (.mp3), which srt to mp3 conversion likely (supposedly) represents. However, you should be able to load these subtitles during media playback, for example in a VLC player. It is worth mentioning that if you would export to a .mp4 file instead, you would be able to embed subtitles in it.
What is possible though, is to use text-to-speech software (Balabolka for example) and generate an audio track based on subtitles. This is useful in case someone wants to create a dubbing for the movie based on subtitles and add that track to the video file using it as an alternative to the original track. You can also embed the original text file into generated mp3 speech file and display subtitles during playback.