Typical of their "do everything approach", the Sennheiser MM 550 will play music whether the NoiseGard cancellation is on or off. The sound changes significant between these two modes, though.
With noise cancellation disabled, these headphones sound great. There's an emphasis on detail that affords them a relatively delicate signature. Bass is well-formed and deep, but it takes a back seat to the upper frequencies. Consequently, there are not masses of slam or attack on offer, but the MM 550 are clearly more "Hi-Fi" than the Bose QuietComfort 15.
They do not supply a great deal of mid-range warmth, however, which can lead to some music sounding a little thin. Regina Spektor's already fragile-sounding voice sounds like it might shatter following a strong gush of wind, for example.
Minor audio problems become far more significant once you turn NoiseGard on. What warmth there is gets sucked-out, leaving the mid-range harsh and jaggedy. Given material prone to harshness, the MM 550 become quite an uncomfortable listen. This is not what we expect from headphones costing more than £300.
The SRS mode doesn't help either. SRS stands for sound retrieval system, and is designed to produce a 3D-like sonic effect with just two speakers. Although somewhat effective in making instruments and voices sound further away from your ear, giving an impression of greater headspace, it also makes music sound slightly compressed. Music largely sounds better without it.
Turning SRS on also introduces some high-frequency noise to the output, which is the last thing these headphones need when the noise cancellation is somewhat noisy.
What the Sennheiser MM 550 get right on the audio front is the use of apt-X. This is an audio codec used to transmit sound over Bluetooth, and it offers better quality than the standard SBC codec. For the two-channel audio here, apt-X transmits at 384kbps - not lossless but not bad either.
With both NoiseGard and Bluetooth enabled, the internal removable battery will last for around eight hours - or around 10 without NoiseGard. The convenience of not needing wires is paid for with having to charge the battery and deal with the idiosyncrasies of Bluetooth.
While you might be able to square away these practical issues, the price is much harder to forget. The Bose QuietComfort 15's £230 price is hard enough to swallow, but when the Sennheisers cost £100 more and yet offer inferior noise cancellation and sound that - while better in many respects - is less pleasant when key functions are enabled, performance simply doesn't justify the outlay.
If noise cancellation is your top priority, the Bose QC3 and QuietComfort 15 cut out more noise. If you want Bluetooth, Sennheiser's cheaper PX 210 BT and PX 360 BT are better value. And if sound's your top concern, you're best off forgetting about extraneous features like Bluetooth and noise cancellation altogether and looking at standard closed-back headphones.
The Sennheiser MM 550 pack in plenty of features - Bluetooth, noise cancelling handsfree calls and wireless music playback controls - but you certainly pay for them. At this price we also expect near-flawless sound to match, but we don't get it. Without noise cancellation engaged they're insightful, if lacking in impact, but NoiseGard introduces mid-range harshness that has no place at this level.