[SalesForce] How to read the value from an input checkbox

Since you are rendering the div inside a loop, you should build the inner portion as separate child component and then pass the param as attribute to the child component. In this way, you don't render multiple div and input elements with duplicate aura ids.

OR

as a work around you can do this:

Leverage the labelClass attribute of inputCheckBox, I am assuming you are not using it. The trick is to have the same ID as the div id in the labelClass of inputCheckbox, so that you get its value when the inputcheckbox is clicked, by which you know under which div the input check box was clicked.

<aura:iteration items.... var="obj">
<!-- div is will be like divId006762hdHGtgjHYU -->
<div id="{!'divId'+obj.Id}" class="slds-button slds-checkbox--button background-silver">
  <label class="answears slds-checkbox--button__label">
    <ui:inputCheckbox label="label" click={!c.voidJS}" class="line" labelClass="{!'divId'+obj.Id}"/><!--Use Id or Use any property of obj which is unique. Should be same as div ID -->
  </label>
</div>
</aura:iteration>

I am using Id field of your loop variable, you can use any unique field value.

Next in your voidJS method,

var clickedElement = event.getSource().get('v.labelClass');
$('#' + clickedElement).removeClass('background-silver');
$('#' + clickedElement).addClass('background-red');

Again, this might not be the most appropriate way of doing this but it works!

First, while XML requires closing tags, HTML does not; by placing the label in the input, you're messing things up. Instead, close the input using />. Second, you should use the "onchange" event.

The following code works in my developer org:

<aura:application extends="force:slds">
    <div class="slds-form-element">
        <div class="slds-form-element__control">
            <span class="slds-checkbox">
                <input type="checkbox" onchange="{!c.showMessage}" name="options" id="checkbox-1" value="on" />
                <label class="slds-checkbox__label" for="checkbox-1">
                    <span class="slds-checkbox_faux"></span>
                    <span class="slds-form-element__label">Checkbox Label</span>
                </label>

            </span>
        </div>
    </div>
</aura:application>