It's not just the Ok Google, but your voice can be a bit quiet in calls as well. Couple things you can do, either root the phone and fix the values in the XML file that control the analog mic gain. Or root the phone and install the latest version of regular CM11, which also fix a few other issues. To root the phone you'll be unlocking the bootloader which will wipe the entire device, but the
instruction is here. Or you could just wait until the next official update.
The XML file in questions is: /system/etc/mixer_paths.xml, and you'll need to edit it with a root-permission text editor like the one built into ES File Explorer.
Search for "Analog input devices" in the XML file, you will see this section:
Code:
<!-- Analog input devices -->
<path name="hph-mic">
<ctl name="DEC5 MUX" value="ADC2" />
<ctl name="IIR1 INP1 MUX" value="DEC5" />
<ctl name="DEC5 Volume" value="84" />
</path>
<path name="main-mic">
<ctl name="AIF1_CAP Mixer SLIM TX7" value="1"/>
<ctl name="SLIM TX7 MUX" value="DEC4" />
<ctl name="DEC4 MUX" value="ADC3" />
<ctl name="DEC4 Volume" value="84" />
</path>
<path name="anc-mic">
<ctl name="DEC2 MUX" value="ADC5" />
<ctl name="DEC2 Volume" value="84" />
</path>
Simply change the: value="84" for each of mic to a higher value, such as: value="99". This will make the mic more sensitive. You might have to test voice call to see how the change in value effect the volume of your voice.