Bluetooth headset pairs successfully, but does not play audio (18.04)

Question

I'm trying to connect a bluetooth headset (Insignia NS-CAHBTEB01-B) to my computer, which is running an up-to-date install of Kubuntu 18.04. Everything seems to go smoothly at first -- I am able to pair the headset successfully using the "Bluetooth Devices" System Settings module GUI -- but the headset doesn't play audio output from the computer. Some things I have noticed:

• I can tell the headset has paired successfully because I hear "Your device is connected" through the headphones. Also, pressing the play/pause button while watching a video in VLC will sometimes cause it to play or pause (not reliably), and using pacmd to change the active profile to headset_head_unit causes the headset to vibrate (see below).
• The headset shows up in the "Audio Volume" widget in the system tray and in system settings. I can try to adjust the volume -- when I do this, I hear a faint hissing noise through the headphones. It persists for a second or two after I stop adjusting the volume slider, and then cuts off.
• Running pacmd list-cards gives the following output (in part):

    index: 4
        name: <bluez_card.E8_07_BF_02_3C_F1>
        driver: <module-bluez5-device.c>
        owner module: 28
        properties:
                device.description = "NS-CAHBTEB01"
                device.string = "E8:07:BF:02:3C:F1"
                device.api = "bluez"
                device.class = "sound"
                device.bus = "bluetooth"
                device.form_factor = "headset"
                bluez.path = "/org/bluez/hci0/dev_E8_07_BF_02_3C_F1"
                bluez.class = "0x240404"
                bluez.alias = "NS-CAHBTEB01"
                device.icon_name = "audio-headset-bluetooth"
                device.intended_roles = "phone"
        profiles:
                a2dp_sink: High Fidelity Playback (A2DP Sink) (priority 40, available: unknown)
                headset_head_unit: Headset Head Unit (HSP/HFP) (priority 30, available: unknown)
                off: Off (priority 0, available: yes)
        active profile: <a2dp_sink>
        sinks:
                bluez_sink.E8_07_BF_02_3C_F1.a2dp_sink/#6: NS-CAHBTEB01
        sources:
                bluez_sink.E8_07_BF_02_3C_F1.a2dp_sink.monitor/#9: Monitor of NS-CAHBTEB01
        ports:
                headset-output: Headset (priority 0, latency offset 0 usec, available: unknown)
                        properties:

                headset-input: Headset (priority 0, latency offset 0 usec, available: unknown)
                        properties:

I can then run pacmd set-card-profile 4 headset_head_unit and pacmd set-card-profile 4 a2dp_sink to switch back and forth between the headset_head_unit and a2dp_sink profiles, which seems to be successful (changes the active_profile line above and causes the headset to buzz when I switch to headset_head_unit), but doesn't get me sound out of the headphones.

Solutions I have ruled out:

• Just rebooting the computer doesn't seem to help.
• The bluez, pulseaudio, and pulseaudio-module-bluetooth packages are all installed and at the latest versions. I've also tried installing blueman and pavucontrol, with no immediate results (I got the impression these are just GTK GUIs whose functionality is duplicated by KDE, but maybe I'm wrong).
• I've looked at some posts here (Blueman Protocol not available, After updating to 16.04, bluetooth audio A2DP mode stopped working) and on the Arch Linux forums (https://bbs.archlinux.org/viewtopic.php?pid=1526534#p1526534). These lead me to suspect that the culprit might be something to do with /etc/bluetooth/audio.conf or the PulseAudio module module-bluetooth-discover. I tried following the steps in Rmano's answer at After updating to 16.04, bluetooth audio A2DP mode stopped working, but it didn't work for me. Some people also suggested switching to HSP/HFP mode, disconnecting, reconnecting, and switching back to A2DP mode, but I think this is the same as what I did above with pacmd, which didn't work.

The only evidence I've found of something going wrong, other than the lack of audio playing through the headphones, is the fact that running pactl load-module module-bluetooth-discover gives the error Failure: Module initialization failed. I'm not sure what's causing this.

How can I get my headset to play audio?