Complex sentence: Frommermann himself was a capable tenor, with a mellifluous speaking voice which he used very effectively and confidentially from time to time; he also supplied most of the bizarre vocal interjections (and stage antics) that decorated the more lively songs.
Split into simpler sentences: Frommermann himself was a capable tenor, with a mellifluous speaking voice, which he used very effectively and confidentially from time to time. Frommermann supplied most of the bizarre vocal interjections (and stage antics) that decorated the more lively songs.