xls to mp3 conversion seems to be highly unusual, but apparently not entirely impossible. Nonetheless you can find websites such as Tripd, that allow users to process properly formated Excel spreadsheets (.xls, .xlsx) and transcript the audio to MP3 format, thus basically making spreadsheet to audio conversion possible. Expect for that, there doesn't seem to exist any other way how to convert xls to mp3.
Open XLS file Open MP3 fileAn online service for translation and transcription of text files