Are you automating those band pass filters? The filters must move over time to get voice sounds.
What software are you using? I achieve vowel like sounds in ableton by taking a synth and running it through two parallel chains, each with single band pass on them. One cut off should be in the low freqs like around 350 hz, and the other in the high freqs around 12khz. Then I map each cut off to the same knob with inverted mapping and ranges that range from their start values to near middle.
So when the control knob is at position 0/100, both band pass are at their respective starting freq (mentioned above at 350hz and 12khz). When the control knob is at 100/100 both bandpass are near mid values, maybe like low = 1khz and high 2.5khz.
You have two play with different values, but the key is as follows.
- Two bandpass filter
- one set high, one set low
- move in opposite directions
Then you take this effect chain and drop it on another chain so this filter effect is only applied about 50% wet.