If you insist...
Sometimes I wonder why you don't worship on Saturday. After all, the Bible does not explicitly command Sunday worship.
Your arguments against instruments (and uninspired music) in worship sound much like the SDA's arguments against Sunday Sabbath.
https://www.amazingfacts.org/media-library/book/e/37/t/is-sunday-really-sacred-
Tim, the RPW is clear. I really can't tell if you follow the RPW by your use of this kind of argumentation. It is by direct command or by good and necessary consequence can be deduced from Scripture or by clear example (in God's public worship). Those things and those things only must we do and can we do in His worship. You seem to be cherry-picking facts from Scripture.
1) We worship on Sunday the Lord's day because of good and necessary consequence and due to clear example from Jesus Himself and the early church.
2) Back to instruments. There are a variety of instruments used, you can categorize them however you'd like, but this one truth and one truth alone you should know. That instruments were introduced only in the worship via the temple (i.e. ceremonial law) in the OT. You don't find them outside of that in public worship at any point in all of Scripture. It is always with reference to the priesthood (Levitical).
Further, there is no NT command, nothing by good and necessary consequence, and nothing given by example in the NT that would lead anyone to come to the conclusion that we ought to use instruments in public worship.
Next, Bill G is correct. Our God not only commands the elements of His worship and what is to be done but how to do it and with what. If you are committed to the pro-instrument position, then it is required in accord with the RPW that you use the instruments He has commanded. You don't get to choose your own or how you want to use the instruments, the instruments the Lord commands or can be deduced from good/necessary consequence must be used (not can be used, they must be used). In fact, you must be using the instrument of your heart, that is clear (Eph. 5:19).
Finally, Tim you stated,
If microphones are a circumstance of worship because they aid in speaking/hearing, why not instruments which are an aid to singing?
In this case, microphones are not required either. Just get everybody really close or have speakers with loud voices as they've done for the past 400 years.
Microphones aren't required. I'm happy you finally see this. For some they are needed as an aid to speaking/hearing. But the use of microphones as aides in worship, or lighting, or air conditioning for comfort, or the time to gather for worship on the Lord's day for being organized and orderly, while they are circumstances that doesn't prove then that instruments are circumstances as well. Because these things (e.g. microphones, etc) are not referred on in Scripture at all, therefore they are circumstances. Your 'why not' question is verging on Lutheran or normative principle thought. Instruments are specifically referenced in Scripture, they were required under the ceremonial law. For this reason alone they can't be circumstances of worship. The ceremonial law has been abrogated having been fulfilled in Christ, therefore instruments are not to be used in public worship today. Further, there is no command or anything by good and necessary consequence that would lead us to believe that we must use instruments in worship today from the Scriptures. If we are to use instruments in worship, then to be consistent, we must be sacrificing animals on the altar as well.
P.S. The OPC's Directory states,
In matters of circumstance and form in worship not specifically provided for in Scripture...
Circumstances aren't referred to in Scripture.
...the circumstances and forms of worship are consonant with God's Word...
Circumstances are consonant with God's word.
Moreover, in ordinary circumstances they are properly administered only in a gathering of the congregation for the public worship of God, baptism being a sacrament whereby the parties baptized are solemnly admitted into the visible church, and the Lord's Supper signifying and sealing the communion of believers with Christ and with each other as members of his mystical body. Nevertheless, if a session judges that circumstances require otherwise, the sacraments may be administered elsewhere; but, in any event, the church must be represented in the service.
Location (Location of worship and where we perform sacraments is not detailed in Scripture whatsoever - thus a circumstance).
So you say that the OPC's view of circumstance is good. I agree, but it appears you do not based in how you have argued your case here.