We differentiate between speaking in tongues in a "loud voice" and praying in the Spirit to God "under your breath", so to speak. This might seem like a fine distinction; but if you have any experience in these things, the difference is really quite clear.
For example, if people are caught up in praise and one man next to me is saying under his breath--or at / below the general level of noise--"Forgive me, Lord..." and the person next to him: "You are Awesome, my God!", and the person next to him: "Shombala kai glossalia", then... what am I doing listening anyway? I should be praising God myself, not eavesdropping on my neighbor! <smiles> For those who get irked at such and trot out 1 Cor 14:27, I say "get a spiritual LIFE!"
But on other occasions, a voice rises up in an unknown tongue at a volume meant to be heard and noticed. This can not be ignored, and thus must be dealt with. (Clearly this will always be the case if the person is on mic, from the pulpit.) For these speaking voice utterances of tongues, "two, or at most three," and "someone must interpret."
The trouble here is, that we must allow for failure... since we cannot know if there will be an interpretation until after the fact . Right? And we must also not ignore the command: "Forbid them NOT to speak in tongues". This is the problem facing us if we wish to do it God's way. We will not generally know in advance if a tongue is going to be interpreted, and dare not forbid the practice.
As a case study of this, there was a brother we knew who spoke out in a LOUD VOICE in an unknown tongue from time to time, and every time no one who would interpret. This dear brother--mature, and dead to self--was very close to the Lord, and I did not doubt the supernatural source of his (very) loud and passionate utterances, but the dead-air afterwards was not the way it was supposed to work. He either caught on, or someone spoke to him about it, and it stopped happening. If a person continues in spite of this, then they should be asked to stop speaking aloud in tongues for a season until there is reason to suspect that someone else in the group might have received the gift of interpretation.
More difficult that the situation where there is no interpretation, in my opinion, is where there is a phony "interpretation". Someone speaks in a (loud) tongue, and this is the pretext for Joe Blow to get up and say what he has been dying to foist upon everyone that no other venue would allow. <sigh> Even Pastors are not immune to this ploy. There was a church outside of the Dallas area where every time someone spoke in a tongue (it seemed) the Senior Pastor would grab the Mic and "interpret" the utterance as being that "the congregation should be giving more money to the church!" <wince> The most absurd example I have come across was a minister Wife/Husband Tongues/Interpretation team where the wife once said about 10-12 syllables of tongues, and the husband took the mic and proceeded to preach a 15 minute sermon. I figure that was about a 1000:1 expansion of words per syllable! <groan> Even his wife was looking askance at him after about 5 minutes of this... "interpreting".
I would rather have to deal with the occasional mistake of exuberance than this kind of deliberate chicanery.
The bottom line is that God did not ask us to be "longsuffering" for drill.