Rambam (Hil. Klei HaMikdash 3:4) lists the instruments accompanying the Leviim:
וּבַמֶּה הֵם מְנַגְּנִין. בִּנְבָלִים וַחֲלִילִים וְכִנּוֹרוֹת וַחֲצוֹצְרוֹת וְהַצִּלְצָל. וְאֵין פּוֹחֲתִין מִשְּׁנֵי נְבָלִים וְלֹא מוֹסִיפִין עַל שִׁשָּׁה. וְאֵין פּוֹחֲתִין מִשְּׁנֵי חֲלִילִים וְלֹא מוֹסִיפִין עַל שְׁנֵים עָשָׂר. וְאֵין פּוֹחֲתִין מִשְּׁתֵי חֲצוֹצְרוֹת וְלֹא מוֹסִיפִין עַל עֶשְׂרִים וּמֵאָה. וְאֵין פּוֹחֲתִין מִתִּשְׁעָה כִּנּוֹרוֹת וּמוֹסִיפִין עַד לְעוֹלָם. וְהַצִּלְצָל אֶחָד בִּלְבַד:
With what did they sing? With lyres, flutes, harps, trumpets, and a cymbal. [The rest of the piece lists how many of each.]
Erchin 11a cites a dispute regarding whether the main part was the instruments or the singing. On the side that the singing is the main part,
וכלי לבסומי קלא הוא דעבידא
The instruments are to enhance the voices.
The Gemara ibid. concludes, based on eleven different sources, that the main part is, indeed, the singing.
Based on this logic, that the instruments are just to enhance the singing, can one add additional instruments to this list?
Perhaps these instruments were the ones selected merely because they were the ones they could make at the time, but if one wanted to add, say, a piano, he would be allowed. Maybe, by the same premise, one could even replace an instrument with another one (though I’m tempted to say otherwise, since there are specifics on how many of each instrument are required).
Or perhaps the list is exhaustive, and there’s something inherent in these instruments that’s pleasing which is lacking in other instruments.
May Mashiach come speedily in our days that this become a practical question.