Creating a new voice for Alexa within my skill

Is it possible to change the voice alexa is using within my skill?

i.e. the user asks

Alexa Ask Car Washing when the next available appointment is?

and have Alexa respond with a voice that matches my carwashing brand?

Yes, it is possible.

You can use SSML Tags in output speech response of your skill to achieve this.

You can

• whisper
• put emphasis on a word or phrase
• use different languages like French, Spanish etc.
• different voices and many more

For Example

<speak>
    Here's a surprise you did not expect.  
    <voice name="Kendra"><lang xml:lang="en-US">I want to tell you a secret.</lang></voice>
    <voice name="Brian"><lang xml:lang="en-GB">Your secret is safe with me!</lang></voice>	
    <voice name="Kendra"><lang xml:lang="en-US">I am not a real human.</lang></voice>.
    Can you believe it?
</speak>

Learn more about Using SSML Tags HERE