ðeɪ ˈædᵻd ðɛɹ ˈoʊn ˌɪnstɹəmˈɛntəlz ɪn ɐ dʒˈæm sˈɛʃən, wˌaɪl lˈiːd sˈɪŋɚ bˈoʊnoʊ dɪvˈɛləpt vˈoʊkəlz æt ðə mˈaɪkɹəfˌoʊn.
They added their own instrumentals in a jam session, while lead singer Bono developed vocals at the microphone.