Because the intended use is for either an earplug or headset, the audio level is intentionally lower than the level coming out the speaker. An amplified speaker may work best depending on you monitoring situation. If you're in your car and you have an auxiliary input jack on your broadcast radio, you can send the audio to your car speakers. That should be enough amplification for you.
I don't know if it works with the Pro 106 but it does with the GRE PSR 500. Take a thin wire and wrap it around the antenna post. Wrap the other end around a non amplified speaker's cable input post and plug it into the radio's headphone jack. This defeats the low volume facor and then a non amplified speaker works perfect. Been using this for 8 years on the 500 without an issue.
Since "best speaker" was used in this thread, I believe this speaker might deserve a review. I believe it works with scanners. Yes, it is a bit pricey, but I've heard some people love them. Gap HEAR IT dsp speaker Product Reviews
I prefer routing my PSR 500 into my MP3 jack on my Kenwood. Nothing sounds like a scanner running through 600+ JBL watts.
Also, I strongly recommend the following mono earphone for your radio. Since the Pro-106 tends to have a lower volume when compared with the GRE version, some people activate the "audio boost" on the radio. If you are using Win500, it is near the the area where you adjust the alarms & LED behavior.
You used to be able to find these at most truck stops, but for some reason, they are hard to find.
I have a professional grade earphone like the ones LE uses with the clear spiral cord, fits inside the ear canal. While it delivers a solid sound, it is still diminished when compared with the aforementioned earphone. The earphone I recommended is positioned over the ear canal instead of inside. It uses a fairly large speaker as well.