Hello Jacksonn7,
Depending on how you've set up this shortcut and which feature is used to speak that your iPhone is charging, you may be able to adjust the voice by using the information in Adjust voice and speed for VoiceOver and Speak Screen on your iPhone, iPad, or iPod touch.
"With VoiceOver, you can turn on more than one voice, switch between Default and Enhanced Quality (without needing to download the voice again), and change the speaking rate for each voice. To adjust the voices:
- Go to Settings > Accessibility > VoiceOver and tap Speech.
- Tap Add New Language.
- Tap the language and dialect that you want. This will take you back to the Speech screen.
- Tap Voice, then tap the voice that you want to use."
"Choose a voice for Speak Screen and Speak Selection
- Go to Settings > Accessibility and tap Spoken Content.
- Turn on Speak Selection or Speak Screen, or both.
- Select Voices.
- Choose the voice and dialect that you want Speak Screen and Speak Selection to use."
Let us know if this helps.
Regards.