The easiest way would be to apply conditional formatting.
Conditional Formatting- Highlight applicable range >> B4:B9
- Home Tab >> Styles >> Conditional Formatting >> New Rule
- Select a Rule Type: Use a formula to determine which cells to format
- Edit the Rule Description: Format values where this formula is true: =RIGHT(B4,3)<>"mp3"
- Format… [Number, Font, Border, Fill]
- OK >> OK