To find the percentage of members in the Jazz Band who can play more than one instrument, you can use the following formula for percentage:
\[ \text{Percentage} = \left( \frac{\text{Number of members who can play more than one instrument}}{\text{Total number of members}} \right) \times 100 \]
In this case, the number of members who can play more than one instrument is 34, and the total number of members is 40.
Substituting the values into the formula:
\[ \text{Percentage} = \left( \frac{34}{40} \right) \times 100 \]
Calculating the fraction:
\[ \frac{34}{40} = 0.85 \]
Now, multiply by 100 to convert to a percentage:
\[ 0.85 \times 100 = 85 \]
Thus, the percentage of the Jazz Band members who can play more than one instrument is
\[ \boxed{85%} \]