Same problem here with Rythmbox and VLC: sound applet doesn't fetch informations from DBUS MPRIS.

I found an error in cinnamon log which is related to the problem:
(cinnamon:2109): GLib-GIO-WARNING **: Dropping signal PropertiesChanged of type (sa{sv}as) since the type from the expected interface is (a{sv})

PropertiesChanged is the signal raised by the player when the song changed (for example).

As far as I see in d-feet & dbus-monitor, everything is OK on the player side (DBUS MPRIS methods are working when called manually, and the PropertiesChanged signal contains everything the applet needs) so the problem is probably on the applet side.

Here is what I see in dbus-monitor when the signal is raised:

-------------------------------
signal sender=:1.53 -> dest=(null destination) serial=178 path=/org/mpris/MediaPlayer2; interface=org.freedesktop.DBus.Properties; member=PropertiesChanged
string "org.mpris.MediaPlayer2.Player"
array [
dict entry(
string "Metadata"
variant array [
dict entry(
string "mpris:trackid"
variant string "/org/mpris/MediaPlayer2/Track/2000"
)
dict entry(
string "xesam:url"
variant string "file:///home/abazigal/Musique/The%20Cranberries/Promises.mp3"
)
dict entry(
string "xesam:title"
variant string "Promises"
)
dict entry(
string "xesam:artist"
variant array [
string "The Cranberries"
]
)
dict entry(
string "xesam:album"
variant string "Stars: The Best of 1992-2002"
)
dict entry(
string "xesam:genre"
variant array [
string "Alternative & Punk"
]
)
dict entry(
string "xesam:comment"
variant array [
string "00003AA0 00000FCC 00016A67 0000AC7C 000118EB 00010D7D 00008B79 00008A28 0000F73E 00004A49"
]
)
dict entry(
string "xesam:albumArtist"
variant array [
string "The Cranberries"
]
)
dict entry(
string "xesam:musicBrainzTrackID"
variant array [
string "5d3388d2-39f7-4c95-8d8d-1232c3816784"
]
)
dict entry(
string "xesam:musicBrainzAlbumID"
variant array [
string "7bc890af-a4a0-4af1-b866-c969852f5353"
]
)
dict entry(
string "xesam:musicBrainzArtistID"
variant array [
string "c98d40fd-f6cf-4b26-883e-eaa515ee2851"
]
)
dict entry(
string "xesam:musicBrainzAlbumArtistID"
variant array [
string "c98d40fd-f6cf-4b26-883e-eaa515ee2851"
]
)
dict entry(
string "rhythmbox:artistSortname"
variant string "Cranberries, The"
)
dict entry(
string "xesam:audioBitrate"
variant int32 162816
)
dict entry(
string "mpris:length"
variant int64 212000000
)
dict entry(
string "xesam:trackNumber"
variant int32 11
)
dict entry(
string "xesam:discNumber"
variant int32 1
)
dict entry(
string "xesam:useCount"
variant int32 0
)
dict entry(
string "xesam:userRating"
variant double 0
)
dict entry(
string "mpris:artUrl"
variant string "file:///home/abazigal/.cache/rhythmbox/album-art/000001ca"
)
]
)
]
array [
]
-------------------------------

My installation also fails to pick up Spotify when I start it. Doesn't matter if I start it from the menu or the applet. The applet hangs, recovers itself on a very rare ocassion...

It's an upstream bug, please follow the upstream bug report: https://github.com/linuxmint/Cinnamon/issues/1962

None of the upstream branches seem to have the code that can be found in the Arch package (they use the old DBUS implementation)... Their code works with the old GJS package but not with the current Arch GJS release...

They maintain the GNOME 3.8 compatibility code here: https://github.com/linuxmint/Cinnamon/pull/1888
We applied the same patch in Arch.

I disabled MPRIS support in cinnamon 1.8.8-3 as it's broken. I don't think that it will be fixed until the next major release (2.0), so I'm closing this bug.