Whether you wish to enhance audio quality or not, either way you need to create a new .wav file with samples from the original one. If you want to keep the quality of the original file, you can write every sample twice.
However there is a rather simple algorithm for "improving" quality: read three consecutive samples, let's name them a, b and c for now. Let z be an extra output sample between a and b.
Let z=(a+b)/2. Now replace b with (a + 2b + c)/4. Remember to calculate z using b's old value! Now only write a, z and the new b to the output.
Read one more sample, let's call it d. Calculate the next extra output sample, y, using (b+c)/2. Use b's old value. Recalculate c, let it equal to (b + 2c + d)/4. Write b's new value and y to the output.
You can figure out the guess, read and repeat while !EOF. Remember to calculate using old values, but output new values.