Hi Jaynewal,
You'll want to adjust the volume of your iPhone while the directions are being spoken to increase the volume. Here are how you adjust additional Spoken Directions settings in Maps:
"Change other settings for spoken directions
Go to Settings
> Maps > Spoken Directions, then turn on or off the following options:
- Directions Pause Spoken Audio: Turn on this setting to pause spoken audio (like podcasts and audio books) when you receive spoken directions.
- Directions Wake Device: Turn on this setting to wake the iPhone display when you receive spoken directions while following driving or cycling directions.
- Directions on Radio: On supported cars, turn on this setting to hear spoken directions when you’re listening to the radio."
You can read the full resource here: Change settings for spoken directions in Maps on iPhone
Additionally you may find some information from this page helpful as well: Get driving directions in Maps on iPhone
Take care.