There is a beautiful explanation of this Gemara given by the Maharal of Prague (in his Chidushim on Agados of Shas) which can help us answer your question.

1. The Maharal asks: What does it matter that the tongue is lying down? Does this stop one from talking? Moreover, do the two walls on either side of the tongue really prevent speech? The Maharal writes that one should know that the words of the Sages are very deep. He explains that the tongue is the only organ in the body whose entire purpose is to move something outwards towards someone else. The ears absorb sounds. The eyes receive visual images. Only the tongue has the role of broadcasting speech to the world. Since the tongue is so outgoing it also carries with it a danger of misuse, of speaking Lashon ha'Ra.

2. Hash-m created every person in a totally complete way, lacking nothing that will prevent him from avoiding negative things. Accordingly, He created the tongue in such a way that if its owner wishes, he can totally avoid the dangers of bad speech. This is the idea of the tongue being created lying down; this symbolizes that the tongue should remain where it is and seek rest in the way that a person lies down when he wishes to rest.

3. In addition, the tongue was created in a hidden place inside the mouth. Certainly this does not prevent a person from speaking. (We now understand why the guards that Hash-m made do not prevent us from learning Torah, in the same way that they do not make it physically difficult to speak to ensure our daily physical needs.) However, what this quality does do is represent the fact that if we want, we can decide not to use the tongue for negative purposes. The tongue was not created in a way that makes it necessary to reveal it, so if we are determined to do so we can make sure that the tongue does not "go out" to say things that it should not.

4. The Maharal adds that the fact that the tongue is flat and is inside the body teaches something about the elevated spiritual level that it is capable of attaining. The fact that it is lying down and supported by other parts of the mouth symbolises its lofty nature, since a distinguished personality of important status is the one who is carried by servants. Furthermore, the fact that the tongue is hidden inside the body demonstrates its superior inner qualities since the nature of spiritual organs is to be hidden inside the body. In contrast, the physical organs are located on the outside of the body.

5. In summary, the guards on the tongue do not physically prevent it from speaking. Rather, they show that the tongue is essentially an inner, spiritual organ. Therefore, if a person so desires, he can decide not to send his tongue out speaking Lashon ha'Ra but rather reserve its extroverted nature for talking Torah.

Kol Tuv,

Dovid Bloom