All Trill notations, including lines, should produce an actual trill sound

What version of MuseScore are you using?

How should MuseScore find out how the trill should be played? i believe the "side note" of a trill depends on the scale (major/minor), which is not available yet in MuseScore. There are also differences with music style etc.

The NotationPlayer is a midi player, he does not interpret notation. Its the opposite.

Trills do depend on the scale, and there are also trills with accidentals that can change that (e.g. placing a sharp or flat or other accidental before the trill mark). If the user manually specified the side note in a trill properties dialog, would that be more doable in the interim while we wait on the needed scale support (and if the user wanted no accidental shown they could set it to be invisible). Furthermore, if a trill properties menu/dialog were added, more specific controls could be added like support for a turn appended to a trill, which is fairly common, and other more advanced features like trill speed, etc.

Also, is there a bug number for the scale support question?

See this too.

How do you do the side notes in the properties? Like, how do you get it to work?

I second--or maybe first as it may be--as I asked for the ability to attach accidentals to ornaments over a year ago. It wouldn't have to be obtrusive, just a little right-click, ornament properties... and up pops a box with appropriate parameters.
Suggestions: For trills have a check box for "start trill on aux note" checked by default. For turns, select whether it is an up turn or a down turn and whether it starts on an aux note or the main note. Also turns should have accidentals available both above and below.
I am not sure if I understand the difficulty with the side note being determined by a particular scale; shouldn't it be just the next note up that is in keeping with the key signature? Even a measured trill of 16ths would be better than nothing, even if it doesn't follow all the rules correctly.

Yes, trills and grace notes should play. Don't make it too "smart" by assuming a scale; just let the user decide on the side note including accidentals if needed, which note to start on, and "how fast," i.e. the trill note values. As an alternative method, I believe Noteworthy Composer had a "layer" feature so you could write them out explicitly but put them in a hidden layer. In that case, the hidden layer was played and the displayed layer muted, which is troublesome as you then have to create both layers throughout the piece.

Although there should be better defaults, see #13811: Select playback method of Grace Notes, Arpeggio & Glissando and Articulations & Ornaments

Please take a look at this forum post.
https://musescore.org/en/node/56281
Let me know you feedback about trills and other ornaments.

See https://github.com/musescore/MuseScore/pull/1946

Still missing playback for trills from the Lines palette.

??? Works fine for me both in 2.0.2 and in development builds. Do you have a specific score and steps to reproduce the issue?

Oops, sorry. It was the case for a while, and I wasn't aware when it was implemented. Just checked it and it works.

Automatically closed -- issue fixed for 2 weeks with no activity.