Speech Synthesis to showcase how various voices sound with System.Speech.Synthesis
Take a look at arrays. It will allow you to quickly add and remove future options without needing to change the logic of the code. It will also allow you to possibly add more complex logic in a single place, instead of having to add code to every if/switch statements.

string[] options = { "Microsoft Hazel Desktop",
                     "Microsoft Heera Desktop",
                     "Microsoft David Desktop",
                     "Microsoft Zira Desktop",
                     "Microsoft Haruka Desktop",
                     "Microsoft Heami Desktop",
                     "Microsoft Huihui Desktop",
                     "Microsoft Tracy Desktop",
                     "Microsoft Hanhan Desktop"};

try
{
    synth.SelectVoice(options[comboBox.SelectedIndex]);
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}