There are two ways to adjust the volume of the injected audio.  The first is to adjust the volume on the injector itself.  To do this, access the streaming PC in the station's local engineering rack and increase or decrease the volume setting on the Injector itself as depicted below.

If that does not normalize the volume to an adequate level you will need to adjust the volume on the sound card itself.  Again, from the streaming PC, access the System's Control Panel > Sounds and Audio Devices.  Then, from the  Audio tab adjust the Sound recording card by clicking the volume button and adjusting the slider accordingly.  If the injected audio (the commercials) is lower than the on air stream then you'll want to pull down the recording card slider until that audio levels between the on air audio and the Injected audio are even.  If the injected audio is louder than the on air audio then you'll want to increase the slider so that you bring the on aid, or incoming audio, up to the level of the commercials.

If you have adjusted the slider on the card as much as possible, and you still have unequal audio, the next step would be to have engineering adjust the incoming audio from the feed source.

NOTE: Most streams are running a bit behind the on air signal due to buffering.  So, it might take 15-30 seconds or so before you can physically hear any adjustments you make to the audio slider.  The best way to work through this is to make an adjustment and wait 30 seconds until you hear it.  Then make another tweak and wait another 30 seconds.  All told you should be able to raise or lower the volume of the entire stop set in about a minute or two.