Just as it is a bit of a half-way house in design terms, the Sony RDP-M15iP is also a mixed bag sonically. The dual drivers give the dock a decent sense of scale and decent volume for a battery-powered device. As is common with smaller docks, the sound becomes slightly strained at high volumes, but this can be compensated-for somewhat with the on-box sound settings.
There are three sound modes available here, selected using a button on the top. The standard flat frequency response mode is on by default, but presses on this button activate the bassier and more high-end focused modes. You can tell which mode is on by the indicator light on the button. An orange light denotes the bassy mode and a green one the treble setting.
The treble mode gets a bit brash-sounding at high volume, but gives the most detailed listen at lower volumes. The orange mode, by contrast, is immediately the most exciting and the go-to pick for parties and barbeques, beefing-up the sound with an extra portion of bass and low-end warmth. Using this mode, the bass response is very good for a 5cm-thick dock - it may be long, but the RDP-M15iP is pretty thin.
It lacks high-end fidelity though. Decent home docks in the £100-150 price, like the JBL OnBeat, offer enough high-end detail and insight to give a near-audiophile sound at low volumes. The Sony RDP-M15iP, though, sounds much less refined, left with a comparatively "2D" representation of music. This is one of the key differences between low-end and mid-range audio tech - the added dynamics and sonic sophistication is rather like adding another spatial dimension to the sound, and you're stuck in 2D territory here.
If you need a home dock and nothing more, you'd be better off with something other than the Sony RDP-M15iP. The wireless, and rather charming, Roth Charlie 2.0 is available for as little as £40 and provides better sound quality, as does the £100 Klipsch iGroove. We're loath to forget the internal battery of the Sony though, since it's a very handy feature for a dock of this size - one that's also capable of blasting out music at a decent volume. If you're going to spend most of the time running the speaker off its battery rather than the AC adaptor though, it's a handy, relatively-high quality portable solution.
With a clever design and decent built-in battery, the Sony RDP-M15iP is the perfect device to take out to a park with friends, or perhaps on a camping holiday. If the battery doesn't mean everything to you though, better sound quality is available for the same price. It'll pump out tunes at a decent volume and has good bass presence for its size, but lacks musicality and detail.