Establish that they can use speech

If the application is speech enabled, make sure that your opening prompt conveys that upfront. Have the first interaction ask them to answer a yes/no question, for instance, by explicitly saying, “you can say ‘yes’ or ‘no’”. However, make sure that you don’t explicitly announce to the user that the application is speech enabled. Chances are that, whatever formulation you choose to make that announcement, some people will not understand what it means.