After some experimentation with a Bluetooth portable speaker on my Spark, and another Bluetooth amp and some speakers in my shop, I've set on a plan for my Spark's stereo system.
I've found it's best to get the speakers as close to your ears as possible to overcome wind noise at speed. The handlebars are about as close as you can get. I'm using a pair of PQN Enterprises SPA22-4BK marine two-way speakers rated at 25W each. I got a pair on eBay for $24 with shipping. Since they are small, they don't have a lot of booming bass, but I'm OK with that.
I'm using an inexpensive Bluetooth amplifier that I also purchased on eBay for $14 with shipping (from China). The amplifier uses an STMicroeletronics TDA7492P chip, which is a BTL (bridge-tied load: one amplifier for + and one for -) class D (switching) amplifier also rated at 25 W per channel. It was designed for LCD TVs. It's a really efficient amplifier chip that requires no heat sink.
The amp board includes a Bluetooth module (the blue board in the photo). There are on-board switches to control Play/Pause, Volume Up and Down, and Next and Previous. I don't use the latter two, so I'm only wiring up three external switches in parallel with the on-board ones. The on-board switches will be inaccessible, as the amp board will be put into a waterproof case located in the area under the gauge.
I'm using some inexpensive waterproof push button switches to control the system. I decided to mount these on the small panel that encloses the space for the optional manual reverse.
This will be a really clean installation. It will be easily reversible, as the handlebar panel and reverse cover panel can be replaced for not a lot of money. I'm going to tap into accessory power at the gauge, so no extra cables or connectors will need to be disconnected to remove central body for maintenance. I'm waiting on waterproof connectors which will be used to connect power, the speakers, and the switches to the amplifier for ready servicing.
I'll post additional details as I complete the installation.

I've found it's best to get the speakers as close to your ears as possible to overcome wind noise at speed. The handlebars are about as close as you can get. I'm using a pair of PQN Enterprises SPA22-4BK marine two-way speakers rated at 25W each. I got a pair on eBay for $24 with shipping. Since they are small, they don't have a lot of booming bass, but I'm OK with that.

I'm using an inexpensive Bluetooth amplifier that I also purchased on eBay for $14 with shipping (from China). The amplifier uses an STMicroeletronics TDA7492P chip, which is a BTL (bridge-tied load: one amplifier for + and one for -) class D (switching) amplifier also rated at 25 W per channel. It was designed for LCD TVs. It's a really efficient amplifier chip that requires no heat sink.
The amp board includes a Bluetooth module (the blue board in the photo). There are on-board switches to control Play/Pause, Volume Up and Down, and Next and Previous. I don't use the latter two, so I'm only wiring up three external switches in parallel with the on-board ones. The on-board switches will be inaccessible, as the amp board will be put into a waterproof case located in the area under the gauge.

I'm using some inexpensive waterproof push button switches to control the system. I decided to mount these on the small panel that encloses the space for the optional manual reverse.

This will be a really clean installation. It will be easily reversible, as the handlebar panel and reverse cover panel can be replaced for not a lot of money. I'm going to tap into accessory power at the gauge, so no extra cables or connectors will need to be disconnected to remove central body for maintenance. I'm waiting on waterproof connectors which will be used to connect power, the speakers, and the switches to the amplifier for ready servicing.
I'll post additional details as I complete the installation.