How to increase the volume in speaker using talkie library in arduino uno...?
Your sample values (that is, the speech[] array) range from 0x00 to 0xfe, using 99.6% of the total range of possible values.

Thus, there is no software solution to increase the volume, as you are only 1 bit away from the maximum output level (0xff is the max, and one less is 0xfe).

This requires a hardware solution. One easy way is to use a commercial audio amplifier or a powered speaker (a speaker with a built-in amplifier).