Good question, good follow-up ...I'd not noticed this before, but CMK's observation about Paul's Judean heritage and the possibility of this being a case of parallelism is interesting.
λαλοῦντες ἑαυτοῖς ψαλμοῖς καὶ ὕμνοις καὶ ᾠδαῖς πνευματικαῖς, ᾄδοντες καὶ ψάλλοντες ἐν τῇ καρδίᾳ ὑμῶν τῷ κυρίῳ, (RP2005)
5:19* speaking unto~yourselves [in] psalms, and hymns, and spiritual songs; singing and psalming in with~the heart of~yours unto~the Lord; (~robin)
If we were to view this as two parallel, but alternating halves:
a) speaking unto~yourselves [in] psalms, and hymns, and spiritual song
b) singing and psalming in with~the heart of~yours unto~the Lord
Then, there's the noticeable balance of:
1) unto~yourselves
2) unto~the Lord
And the additional balance of:
3) speaking [in] psalms, and hymns, and spiritual song
4) singing and psalming in with~the heart of yours
speaking to one another in psalms and hymns and spiritual songs,
singing and making melody with your heart to the Lord; (NASB)
If the question is ...
Q1: Does singing and making melody (psalming) ... mean different things?
Then, if this is an intended parallelism, shouldn't we also be asking ...
Q2: Does speaking in psalms, hymns, and spiritual song ... mean different things?
Also, notice (the asterisk) that there are source text variations in this verse; that is, the preposition "in" is present before "psalms" is some texts, and not in others ... And the "in" before "the heart" is present in some, and not in others (see the variants list below).
The reason I mention this is that this proposition is another dividing point, in both of our parallel halves ...
3) speaking [in]
psalms, and hymns, and spiritual song
4) singing and psalming in
with~the heart of yours
This has the appearance, then, if we continue with the idea of parallelism being involved, of "speaking" being one thing, and alternately of"singing and psalming,"
also, being one thing (so to speak).
And yet, "adontes kai psallontes" are obviously two different words, two different verbs "singing {0103 V-PAP-NPM} and {2532 CONJ} psalming {5567 V-PAP-NPM}" ... the obvious emphasis, here, being that they are, in fact, different, different words that mean different things, different actions.
Q: Can psállō mean "playing an instrument", or "singing" and "making melody with your heart" are used interchangeably to mean "singing"?
So, even though the parallel balancing would group these two, together, over against the opposite side's singular "speaking," that's really not sufficient reason to think of "singing" and "psalming" as a singular/ equal something.
Also, you suggest that "psállō" might mean exclusively "playing an instrument," but you didn't really say how this idea is supported. I'm not sure that one could say "exclusively," but it is associated with "psElaphaO" (to touch lightly, twang, or snap), which does sort of relate to playing a stringed instrument. However, it also is used in a more general way to signify making music in any fashion, so it could extend to "making music" in any manner, such as simply singing praises or psalms to God, whether with or without instruments. [Spiros Zodhiates, The Complete Word Study Dictionary].
Also, I noticed that you interjected the thought that you strongly doubt that God "commands" us to play an instrument, even though the verb is in the imperative tense. Interesting how the idea of "command" comes up here and in another recent question when in neither topic threads, there's any thought of there being a "command" involved in their verses? And just to further clarify, neither verb... "singing {0103 V-PAP-NPM} and {2532 CONJ} psalming {5567 V-PAP-NPM}" are imperative, they are present, active, participles [imperative, by the way, is mood, not tense].
And, as long as I'm quibbling, why would the NASB have properly read "psalmois"
as "psalms," but then have gone so far afield and translated "psallontes" with the the poor reading of "making melody?"
lalountes heautois psalmois kai humnois kai Odais pneumatikais adontes kai psallontes en tE kardia humOn to kuriO
5:19* Addition
ψαλμοῖς
א A D F G Ψ 1881 Byz vgms Tertullian ς WH
psalmois
unto~psalms {5568 N-DPM}
*en psalmois
in {1722 PREP} unto~psalms {5568 N-DPM}
ἐν ψαλμοῖς
p46 B P 0278 6 33 1739 pc it vg (NA [ἐν])
5:19* Deletion
ἐν τῇ καρδίᾳ
Ψ 0278 33 Byz ς
en tE kardia
in {1722 PREP} unto~the [one] {3588 T-DSF} unto~a~heart {2588 N-DSF}
*tE kardia
unto~the [one] {3588 T-DSF} unto~a~heart {2588 N-DSF}
τῇ καρδίᾳ
p46 א* B 1739 1881 WH