Multiple microphones and amplifier technology are helping to make phone calls, songs and videos sound louder and clearer for some time. Don't rule out "old school" approaches, however. Falco Freeman has a created a Kickstarter project for the AudioGlove; an iPhone(s aapl) case that uses simple accoustics to boost sound as needed and it clearly works; testing at Dolby's sound labs proved the case, as it were, which should boost sound volume by about 20 percent.
