Can we have other instruments playing during the mass? Is it part of the (taks) of the church or is it more tradition?
Until a little over a hundred years ago, it seems that Coptic liturgies were prayed without the use of even the cymbals and triangles that are common today. Only a set of large cymbals were struck at important points in the liturgy, to draw everybody's attention.
In our African Coptic Orthodox Churches, the Church has shown some flexibility to accomodate their native culture. For example, I once attended a liturgy there where a little boy with a traditional African drum crouched at the end of the choir and accompanied even the Coptic style hymns (alhan). It seems this particular village could simply not envisage singing without the drum accompaniment!
There is of course nothing in the Bible or in our ancient traditions that sets a law about using musical instruments to worship God. In fact, the psalms often mention the use of instruments that were common in those times like harps and cymbals and tambourines. However, our Coptic tradition has favoured focusing on the human voice as the chief instrument of praise. This is a particular musical style and I find it quite beautiful in its rawness and honesty. We offer God not our cleverness or our talent, but our simple selves, our 'naked' voices, lifted up from the heart in humility. Traditional Coptic liturgical hymns are not works of art, they are raw prayers.
In many other traditions where heavy musical accompaniment is used, it seems to me that there is a temptation to be distracted by the music itself and lose focus on the prayer dimension of the exercise. Some may disagree with this of course, and that may come back to a matter of personal taste. An analogy might be how I find it very hard to pray in a cathedral which is very majestic and brimming over with masterpieces and artworks; I pray much better in a simple and bare chapel. The environment can draw your eyes (or ears) to its own beauty and the mind may wander to think about the cleverness of the artist or composer, leading you to forget the actual purpose of the artwork. The Coptic acapella style has far fewer distractions, although of course, one can find distractions if one looks hard enough ("Hey, they missed a note!")
In summary, the lack of musical instruments in the Coptic liturgy is a tradition, not a commandment, but I (and many others) think it is a tradition that has much to recommend it and should be preserved. There is no problem with using musical instruments apart from the liturgy, say at Youth Groups or in Sunday School, so long as the music is just a tool and never becomes a goal in itself.
This Q&A was found on Archangel Michael & St Bishoy Coptic Orthodox Church : http://www.stbishoy.org.au/modules/smartfaq/faq.php?faqid=72