Oregon's hand-free law prohibits/prohibited certain "uses" of mobile communication devices while driving. The law does not define the verb "use"; for which reason, in State v. Rabanales Ramon the court had to discover and clarify the intended meaning of the statute.
State v. Rabanales Ramon (2015)
The court concluded that "ORS 811.507(2) is limited to use of a mobile communication device for the purpose of voice or text communication". Noting that the kind of device in question is "a text messaging device or a wireless, two-way communication device designed to receive and transmit voice or text communication", the court infers that "the activity that the legislature intended to address was using a mobile communication device for voice or text communication". Furthermore,
The legislature did not include in its definition of “mobile
communication device” a device incapable of receiving and transmitting
voice or text communication, such as a hand-held GPS mapping device or
music-playing device, which suggests that the legislature did not
intend to broadly prohibit the use of any mobile device for any
Other details of legislative history of the present and previous laws indicate that
the activities that the legislature intended to address were talking
and texting on mobile communication devices while driving
and the court observes that
although the legislature was aware of various distractions that could
lead to dangerous driving, ORS 811.507 was intended to prohibit a
specific type of distraction while driving—talking and texting on a
mobile communication device
Oregon v. Rabanales-Ramos was decided on August 19, 2015, there is no indication in the appeal when the arrest happened. Footnote 1 says
ORS 811.507 was amended in 2013, with an effective date of January 1,
2014. Or Laws 2013, ch 757, §§ 1, 3. Those amendments do not affect our analysis of this case.
Since the ruling there have been other amendments.
The ruling address
"mobile communication devices", and not "mobile electronic device" which is what the current statute refers to. Compare the two versions:
‘Mobile communication device’ means a text messaging device or a
wireless, two-way communication device designed to receive and
transmit voice or text communication.
(A) “Mobile electronic device” means an electronic device that is not
permanently installed in a motor vehicle. (B) “Mobile electronic
device” includes but is not limited to a device capable of text
messaging, voice communication, entertainment, navigation,
accessing the Internet or producing electronic mail.
We can conclude from this (and no doubt from other legislative records) that in the interim, the legislature changed its intent – it expanded the scope of devices and uses that are prohibited.
There has not been an adjudicated challenge to the interpretation of the current law, but it is highly likely that the courts will now consider "listening to music" to be an example of a forbidden use.
I assume in your question that you are referring to some system whereby content is transmitted to the built-in car speakers. But the built-in status of the speakers is irrelevant, because under your hypothetical, the music comes from a phone which is clearly a "Mobile electronic device". Because of the age of the driver, we needn't decide whether the affirmative defense is available (it isn't) – that is, whether you have described a “Hands-free accessory”, which means
an attachment or built-in feature for or an addition to a mobile
electronic device that gives a person the ability to keep both hands
on the steering wheel at all times while using the device or requires
only the minimal use of a finger, via a swipe or tap, to activate or
deactivate a function of the device.
The well-established and accepted hands-free devices only requires a don't-look tap to turn on / off, which is not the same as tapping various screens on a phone to navigate between albums on an MP3 player. We might well conclude that even an adult is not allowed to use a phone as a music player, whether or not the sound comes from the phone speakers or is transmitted to the car's built-in speakers.