void WaveDAC8_SetValue (uint8 value) This sets the current value. no more.
DAC generate a signal, using a table of fixed values.
Unfortunately, PSOC had eight-bit DAC PSOC. Even when using the whole range (0-255)the signal has the steps of gradation levels.
It makes no sense to reduce table values.
So you have to use a potentiometer (digital potentiometer) at the output of the DAC.
...Hope I'm not mistaken
There are many ways to change signal amplitude. It all depends on required bandwidth, output resolution, gain step size, etc. Please check previous discussion on that (examples are included)
To find best approach you must provide more information on the specs..